Let us attempt to identify where the issue lies...
Which AppInventor server are you using? ai2, code, ai2-test ?
Try with a private browsing window (no extensions) do you get the same behaviour?
Try with Chrome ? (Works for me on Xubuntu 24.04.02 LTS)
[EDIT]
Just tested with ai2 server on Firefox 141 - I see the same behaviour
I have found if I type text into the comment box,leave it open, return to the projects list, then return to the project - the comment text is still there, and can be closed and opened to see the text again. Not ideal...
Here's what I get on Brave, Windows 10, with existing comments from Do It:
What I am seeing is that closed comments, upon return to the Project from the Projects list, are reshaped and have their cursor positioned to the end, presenting the appearance of having lost text until you reshape the comment bubble by dragging its lower right corner.