diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index 72d0c9041e0..a4a1f974d07 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -778,7 +778,6 @@ def __init__(self, pluginmanager, *, invocation_params=None) -> None:
             )
 
         self.option = argparse.Namespace()
-        self._nondefault_options = set()  # type: Set[str]
         self.invocation_params = invocation_params
 
         _a = FILE_OR_DIR
@@ -892,10 +891,9 @@ def _processopt(self, opt: "Argument") -> None:
         for name in opt._short_opts + opt._long_opts:
             self._opt2dest[name] = opt.dest
 
-        if hasattr(self.option, opt.dest):
-            self._nondefault_options.add(opt.dest)
-        elif hasattr(opt, "default"):
-            setattr(self.option, opt.dest, opt.default)
+        if hasattr(opt, "default"):
+            if not hasattr(self.option, opt.dest):
+                setattr(self.option, opt.dest, opt.default)
 
     @hookimpl(trylast=True)
     def pytest_load_initial_conftests(self, early_config):
diff --git a/src/_pytest/nodes.py b/src/_pytest/nodes.py
index 1bc8b9bb393..854b7328f5c 100644
--- a/src/_pytest/nodes.py
+++ b/src/_pytest/nodes.py
@@ -335,11 +335,11 @@ def _repr_failure_py(
             return excinfo.value.formatrepr()
 
         # XXX should excinfo.getrepr record all data and toterminal() process it?
+        # XXX: does not distinguish between default/unset and --tb=auto.
         tbstyle = self.config.option.tbstyle
         verbosity = self.config.option.verbose
         if fulltrace:
-            tbstyle_given = "tbstyle" in self.config._nondefault_options
-            style = tbstyle if tbstyle_given else "long"  # type: _TracebackStyle
+            style = "long" if tbstyle == "auto" else tbstyle  # type: _TracebackStyle
         else:
             if tbstyle == "auto":
                 if default_style is None: