Why does Unsplit Lines create Multipart Features?

[BarryKronenfeld1]

I have a stream feature class with many features that form a chain, one after the other with no side streams coming in. I am trying to create a clean feature class of stream segments by combining segments that meet at their endpoints using the Unsplit Lines tool. The output feature class does combine features, creating just one feature for each section between stream confluences... but the problem is that many of these are now multipart features. Why does this happen - is it a bug?

To make sure that it wasn't a problem with the input data, I first ran the Multipart to Singlepart tool to get a feature class that would have no multipart features as input. I then used the field calculator to confirm that the features each had only one part. I then ran the Unsplit Lines tool on the result and the same thing happened - the output features had multiple parts.  I'm using ArcGIS Pro 2.8.1

[DanPatterson]

Unsplit Line (Data Management)—ArcGIS Pro | Documentation

Did you use a dissolve field or fields?

[BarryKronenfeld1]

No. Good thought, I could see where that would happen with a Dissolve field but no I'm just doing a simple run of Unsplit Lines with no Dissolve field, accepting all default parameters. Input features all have only one part, but output has multipart features.  I'm having to go in and manually explode those features and then merge them back together again.

[BarryKronenfeld1]

OK, so I think I found the culprit - it seems that the input data has overlapping segments. They must be tiny because I can't see them even when zoomed in as far as I can go... The clue is that when I manually "explode" a feature and then "merge" it back together with the defaults, it becomes multipart again, but if I uncheck the box to "preserve overlapping segments" then it gets reduced to a single part.