During the last week, I was receiving notifications about new messages with great delay, multiple at once, the next day after they were sent.

I have an account on dismail.de (Prosody 0.11.1), contact uses wiuwiu.de (Prosody 0.11.2)

My device/system details:
iPhone XS running on iOS 12.3.1
Monal version 3.6 (354)