I copy-past 4 measurers. At the end of bar 4 there are more rests than the original source (see GIF).
I have play around what is happens, add and delete some measures. What you can see is that one measure has 8 beats instead of 4 beats.
shft-b -12 delete all measures, copy paste now working fine.
I can only assume that there was something interesting going on in the destination passage. Without seeing the actual project it’s hard to diagnose the problem in detail.
You have the ZIP file
I have discovered that sometimes an normal barline is removed. Related to another thread from me there is a problem with the Lock Duration tool when the caret is de-active. When selecting content with the LockDuration tool ON it is very simple to make failures. I thing the normal barline is removed and then the measure is 8/4 instead of 4/4.
Note: The Lock Duration tool can’t be set off with the L shortcut when the caret is not active. Only click on the button works. I have a 34" ultra wide screen, so I hope this is solved in the future