I tried a few things, and the aerc behaviour seems fine. When I read an email, the “S” is added, and when I move it, the flag is preserved.

However, I observed 2 scenarios:

• If I read > mbsync > move > mbsync, the result is the expected one, having the mail red and moved on both server and locally;
• If I read > move > mbsync, the mail is red and moved locally, but it’s only moved on the server. This means that the next time that I do mbsync, the mail will revert to unread locally.

One thing I noticed (which might be the problem somehow?) is that:

• The filename is initially something like “1756379541.12762_13.host,U=13:2,”
• After the reading changes to “1756379541.12762_13.host,U=13:2,S”
• After the moving changes to “1756379541.12762_13.host:2,S”
• After the sync changes to “1756379541.12762_13.host,U=3376:2,S”
• And eventually, after another sync the S flag is removed

This means that the difference in the 2 scenarios above, is that in the first case (the expected one) the mail is synced while having the U=13 present, while in the second case is synced with the U removed.
I tried to understand what’s the meaning of that part (which looks like an ID) and if it’s relevant in the process, but I didn’t find any information, sadly.

For now, I think I will just use the imap connection rather than local, but if you have any insights, please let me know! 🙃 And thanks again for the debugging tip, it really helped to understand what’s going on! :D

Thank you very much the info! I was looking for something like this, but eventually I failed 😆
I’ll try as soon as I can and get back with the result!