@subbasshead
I'm having trouble reproducing this on an install running the classic editor plugin and using the Twenty Twenty One default theme.
This is what I have tried:
- Create new post
- Switch to classic editor, text mode
- Add media, change to large and insert
- Create new post
- Switch to classic editor, text mode
- Add media, default size shown is large, change to medium
I repeated the above a few times, each time switching to a different image size and for the next post the default size was showing the previously used version.
Please let me know if I've missed a step or you're able to reproduce the error without plugins.