I think I may have stumbled onto a (recent?) bug which occurs when combining tied tuplets.
In the attached screenshot, I selected the Violin 1 and 2 measures, copied them, then pasted into the empty Viola measure using the “Paste Special -> Reduce” menu item. Despite the violins having the same exact rhythm, the resultant viola measure isn’t uniform. I am definitely selecting the entire violin measures (including the triplet "3"s), so I’m not sure what’s happening.
This also occurs when using “Paste Special -> Move to Staff Above (/Below)” to combine the violins.
Interestingly, this is only a problem when tuplets have one or more tied notes.
Should be pretty easy to replicate this based on the above instructions, but let me know if more info is needed.