I'm using Vegas 16 build 361.
I created a project 4096x3072 and added a single GH5 6K file to it. That worked.
Then I did a 16:9 crop, just the default of full width. After saving the project, I closed Vegas and after a short while got an error that Vegas had crashed. I then reopened the project and removed the crop by resetting it to the default. After saving and closing, Vegas crashed again.
It seems that any crop will cause Vegas to crash on exit. I've reported it.
However, I can still work on the project, render it etc.