Did you verify the integrity of the download?
https://electrum.org/#downloadTry reinstalling electrum and verify the integrity of the package, then try to reproduce this bug and see if it happens again.
Maybe your files got corrupted or something, because it looks like a weird bug.
I did. I downloaded, verified, and reinstalled again before trying to send out some btc to another wallet but it won't happen again. Also I've done 4 transactions (plus many unsigned tx when trying to reproduce it to take a screenshot) after I wrote this thread but never got the same problem again.