Kibana line visualization hover labels showing incorrect values

[IrdmEngr]

Kibana version:
First noticed on 7.5.2, upgraded to 7.6.0 and still seeing it.

Elasticsearch version:
First noticed on 7.5.2, upgraded to 7.6.0 and still seeing it.

Server OS version:
Windows 10 Pro

Browser version:
Chrome Build 80.0.3987.87

Original install method (e.g. download page, yum, from source, etc.):
Download .zip package, unpack and launch via command line

Describe the bug:
When creating a visualization in Kibana, the labels that appear when you hover over points work as expected with a standard line plot. But when you add sub-buckets, the label for the second item is pulling the value from the third item instead of the name from the second (it's confusing to explain, see image)

Steps to reproduce:

1. Create new line visualization in Kibana
2. Add bucket of x-axis for time, y-axis to something like count of events matching filters
3. As a starting sanity check, now hover cursor over a point on the plot. You should see the first items label as @timestamp per day (or whatever your aggregation is), it's value to the right, then the second item down's label should be, in this case, count (let's refer to this as the item of suspicion), followed by it's value to the right. Verify they are as expected.
4. Add a sub-bucket for splitting your lines. For example, by location of hits.
5. Now hover cursor over the a newly updated point. You will likely see the first items label as @timestamp per day (or whatever your aggregation is), it's value to the right, then the second item down's label should still be, in this case, count, with it's value to the right, then third item down would be, in my example, location, then it's value to the right. What you'll likely see instead of "count" for the item of suspicion as the second one down, it appears to be populating whatever value it pulls from "location" value and puts it in the column label for item 2. In my case, instead of the label being "count" it was displaying the location name, followed by the count value to the right.

Expected behavior:
Label should still be "count" or whatever the aggregation selected was. See above for more.

Screenshots (if relevant):

This particular screenshot is counts of occurrances, split in sub-buckets by "dropped" values.
What it should be displaying - The left hand column, second item down, should be labeled "Count" and displaying value of 6, with the dropped.keyword value below being accurately displaying "Y". But instead of showing "Count" it appears to pull the value from column 3 and displays it as the label in column 2. It does this across all of my visualizations that I've made, past and present.

Errors in browser console (if relevant):
None.

Provide logs and/or server output (if relevant):
None.

Any additional context:
Seems to do this on the last several Kibana versions, but only ones I remember for certain were 7.5.2 when I first noticed it, and now 7.6.0 since I've upgraded.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[lizozom]

Could reproduce on 7.6 using the provided steps on the ecommerce sample data set.

Split Series configuration:

The resulting tooltip looks like this:

[flash1293]

Duplicate of #41887