Several folks using dovecot (and others) have run into this bug as reported above. Not specific to attachments, but in general, broken syncing of Apple Mail shipped with iOS 18.x.x - everything is fine with Apple Mail shipped with iOS before iOS 18...and macOS Mail Version 16.0 (3826.300.87.4.3) shipped with macOS Sequoia 15.2...
There is discussion with Stalwart IMAP server a fix they pushed on their code. The issue seems to be that new Apple Mail in iOS 18 is using command pipelining and
https://github.com/stalwartlabs/mail-server/issues/765#issuecomment-23629310...
"Starting with iOS 18, Apple Mail began using a technique called "pipelining," where multiple IMAP commands are sent together in a single network packet. Specifically, after the IDLE command is terminated with the DONE command, iOS 18 was immediately sending other commands in the same packet.
Although Stalwart supports command pipelining, it was not prepared to handle this scenario. When it received the DONE command, it stopped processing the rest of the packet and ignored any subsequent commands that followed. This caused Apple Mail to get stuck, as its subsequent requests were not being processed correctly."
I happen to be using an older dovecot that shipped with macOS server a few years back. Version: 2.2.30.2 Now, from what I've read, IMAP supports command PIPELINING per the IETF IMAP spec from a long time ago.... Is it possible, that like Stalwartlabs, that dovecot is ignoring and commands it receives after the "DONE" command, and not reading any new commands following DONE that appear in the same packet? Could someone with deep dovecot experience perhaps dive into this and even, perhaps find out if it is APPLE who broke the IETF spec, or if it is a bug within dovecot??