Ok so both you and #[2] are saying it happens in Brave. Brave has some quirks about it but I've got most of those ironed out. I'll see if I can put some error trapping in place and catch any failed posts and resubmit them or at least inform you that it failed and ask if you want to resubmit.
Discussion
Alright I added additional error trapping. If a note fails to post after 6 seconds it will show a pop-up notifying you that it failed, attempt to reconnect to the relays and reset the post button (leaving your message in tact) so you can try again. Refresh for the update and let me know if that solves the issue.
As luck would have it I can't replicate now, but I'll let ya know next time I encounter