diff --git a/lib/queue-msg/src/optimize/passes.rs b/lib/queue-msg/src/optimize/passes.rs
index fe98da41eb..fd32083350 100644
--- a/lib/queue-msg/src/optimize/passes.rs
+++ b/lib/queue-msg/src/optimize/passes.rs
@@ -210,7 +210,15 @@ impl<T: QueueMessageTypes> PurePass<T> for FlattenSeq {
             fn go<T: QueueMessageTypes>(msg: QueueMsg<T>) -> Vec<QueueMsg<T>> {
                 match msg {
                     QueueMsg::Sequence(new_seq) => new_seq.into_iter().flat_map(go).collect(),
-                    QueueMsg::Concurrent(c) => vec![conc(c.into_iter().flat_map(go))],
+                    QueueMsg::Concurrent(c) => vec![conc(c.into_iter().flat_map(|msg| {
+                        let mut msgs = go(msg);
+
+                        match msgs.len() {
+                            0 => None,
+                            1 => Some(msgs.pop().unwrap()),
+                            _ => Some(seq(msgs)),
+                        }
+                    }))],
                     QueueMsg::Aggregate {
                         queue,
                         data,
@@ -253,18 +261,16 @@ impl<T: QueueMessageTypes> PurePass<T> for FlattenConc {
             fn go<T: QueueMessageTypes>(msg: QueueMsg<T>) -> Vec<QueueMsg<T>> {
                 match msg {
                     QueueMsg::Concurrent(new_conc) => new_conc.into_iter().flat_map(go).collect(),
-                    // wrap in conc again
-                    // seq(conc(a.., conc(b..)), c..) == seq(conc(a.., b..), c..)
-                    // seq(conc(a.., conc(b..)), c..) != seq(a.., b.., c..)
-                    QueueMsg::Sequence(s) => vec![seq(s.into_iter().map(|msg| {
+                    QueueMsg::Sequence(s) => vec![seq(s.into_iter().flat_map(|msg| {
                         let mut msgs = go(msg);
 
                         match msgs.len() {
-                            // return the original empty sequence
-                            0 => seq([]),
-                            // seq(a) == a
-                            1 => msgs.pop().unwrap(),
-                            _ => conc(msgs),
+                            0 => None,
+                            1 => Some(msgs.pop().unwrap()),
+                            // wrap in conc again
+                            // seq(conc(a.., conc(b..)), c..) == seq(conc(a.., b..), c..)
+                            // seq(conc(a.., conc(b..)), c..) != seq(a.., b.., c..)
+                            _ => Some(conc(msgs)),
                         }
                     }))],
                     QueueMsg::Aggregate {
@@ -298,8 +304,10 @@ impl<T: QueueMessageTypes> PurePass<T> for FlattenConc {
 mod tests {
     use super::*;
     use crate::{
-        data, effect, fetch, noop,
-        test_utils::{DataA, DataB, DataC, FetchA, PrintAbc, SimpleMessage},
+        aggregate, data, defer_relative, effect, event, fetch, noop,
+        test_utils::{
+            AggregatePrintAbc, DataA, DataB, DataC, FetchA, PrintAbc, SimpleEvent, SimpleMessage,
+        },
     };
 
     #[test]
@@ -349,4 +357,72 @@ mod tests {
         assert_eq!(optimized.ready, expected_output);
         assert_eq!(optimized.optimize_further, []);
     }
+
+    #[test]
+    fn seq_conc_conc() {
+        let msgs = vec![seq::<SimpleMessage>([
+            conc([
+                aggregate([], [], AggregatePrintAbc {}),
+                aggregate([], [], AggregatePrintAbc {}),
+            ]),
+            conc([
+                aggregate([], [], AggregatePrintAbc {}),
+                aggregate([], [], AggregatePrintAbc {}),
+            ]),
+            conc([
+                repeat(None, seq([event(SimpleEvent {}), defer_relative(10)])),
+                repeat(None, seq([event(SimpleEvent {}), defer_relative(10)])),
+                // this seq is the only message that should be flattened
+                seq([
+                    effect(PrintAbc {
+                        a: DataA {},
+                        b: DataB {},
+                        c: DataC {},
+                    }),
+                    seq([
+                        aggregate([], [], AggregatePrintAbc {}),
+                        aggregate([], [], AggregatePrintAbc {}),
+                        aggregate([], [], AggregatePrintAbc {}),
+                    ]),
+                ]),
+            ]),
+        ])];
+
+        let expected_output = vec![(
+            vec![0],
+            seq::<SimpleMessage>([
+                conc([
+                    aggregate([], [], AggregatePrintAbc {}),
+                    aggregate([], [], AggregatePrintAbc {}),
+                ]),
+                conc([
+                    aggregate([], [], AggregatePrintAbc {}),
+                    aggregate([], [], AggregatePrintAbc {}),
+                ]),
+                conc([
+                    repeat(None, seq([event(SimpleEvent {}), defer_relative(10)])),
+                    repeat(None, seq([event(SimpleEvent {}), defer_relative(10)])),
+                    seq([
+                        effect(PrintAbc {
+                            a: DataA {},
+                            b: DataB {},
+                            c: DataC {},
+                        }),
+                        aggregate([], [], AggregatePrintAbc {}),
+                        aggregate([], [], AggregatePrintAbc {}),
+                        aggregate([], [], AggregatePrintAbc {}),
+                    ]),
+                ]),
+            ]),
+        )];
+
+        let optimized = Normalize::default().run_pass_pure(msgs.clone());
+
+        assert_eq!(optimized.optimize_further, expected_output);
+        assert_eq!(optimized.ready, []);
+
+        let optimized = NormalizeFinal::default().run_pass_pure(msgs);
+        assert_eq!(optimized.ready, expected_output);
+        assert_eq!(optimized.optimize_further, []);
+    }
 }