I use version 12.1 on android and set-up an IMAP-account. However, if I delete it on my phone it is not deleted on the server. The relevant settings are ticked. I use kreativmedia.ch as e-mail provider.
I’ve found following error-message in the log file:
This is due to a bug introduced in version 12 releases.
To workaround this issue, go to Settings > Account Settings > Fetching mail > Incoming server, disable the “Auto-detect IMAP namespace” and leave the IMAP path prefix empty.”
I’d also recommend checking special folders such as archive, spam, sent, trash etc. are still mapped correctly under settings > account > folders.
See github ticket #9567 for more detail. A fix has already been applied, and being tested in beta before wider rollout.