Apple Mail: Erase deleted messages [from Trash]: After one week Fails: acts like Never

Mail 10.3 running in macOS 10.12.6 Sierra on a Mid-2012 15" Non Retina MacBook Pro.

Email Service Provider (ESP): Fastmail.com, using IMAP

Woke up this morning to a message from Fastmail that my account was overfull, and i was losing any new incoming email. I always use Mail, not their (or anyone’s) WWW interface. I only use this one instance of Mail on this one Mac, not other devices (rare use of old iPhone 4S over WiFi, a couple of times a year, with that system’s Mail and this email account).

Erase deleted messages has been and remains set to After one week for this account.

I found deleted (in Trash) emails going back to about February of this year.

How can this be? What do i need to do to resolve this issue?

Understood that the server is in control. But if things are working properly and Mail determines that it’s time to actually completely delete a message as it’s older than one week, shouldn't it be able to send this deletion signal to the IMAP server and make it so?

This same failure is happening on another email account, same Mac, same Mail, same ESP. Anecdotal report that it is not happening—working correctly—on a different Mac (Mid 2011 MacBook Air) with another account at the same ESP (Fastmail). All 3 accounts are domain email accounts under a domain i control.

Thanks in advance for any assistance. Hoping to permanently resolve this issue and not lose any more email.