aerc mail client + mbsync problem (maybe)

Dear fellow nerds,

I recently started to use aerc, synched locally with mbsync. Unfortunately, I'm facing a rather annoying problem with Gmail:

• I read emails (flag "seen") and move them to the archive;
• run the mbsync < account > command to sync;
• the mails are correctly moved, however, they seem to lose the "seen" flag, and they revert to unread.

Am I missing something? Any idea what could be the problem? If it matters, I'm on a Wayland session on NixOS.

Thanks!