I may be missing something, but I cannot seem to do anything with scattered grass once I exit a session. I can edit just fine up until the point I try to revisit.
I have searched around quite a bit and have not seen anyone else talking about it.
Are you still having issues with this. I just tested and wasn't able to replicate any issues with this, and I was able to edit scattered vegetation in a previously saved session.
If it's still happening, a video demonstrating the problem might help me better understand the issue.
Have the same thing with painted grass from previous session. Next day it freeze Twinmotion process while exporting images. I tried to change dencity and remove some grass with eraser, but it did not work. So, I think there is some issue with foliage tools.
If you can submit a bug report at https://twinmotionhelp.epicgames.com/s/case-community-page with the file containing this foliage, our support team can investigate further.