IMAP push stopped working?

Recently I noticed I haven’t been getting push notifications for messages when my phone is idle or DC is backgrounded, so it seems IMAP push stopped working for me on Android. I don’t know exactly when this started happening. IMAP push worked fine for me in the past. I haven’t enabled “force background connection” setting but I didn’t need this in the past.

Has anyone else had an issue with this? I haven’t changed anything on my phone, but I can think of at least four possible reasons:

  1. New bug in update to DC app
  2. New bug in nine.testrun.org server
  3. New bug in update to my VPN
  4. New bug in update to my OS

Unfortunately it’s hard to know the exact cause for this problem, which is why I ask if anyone else has similar issue or is it still working fine for everyone else who relies on IMAP push (not FCM) and using nine.testrun.org?

I think I maybe discovered the cause of the problem which was something else I didn’t think about, a change to my firewall settings.

Is there any IMAP push test tool that could be used to verify if an IMAP push is working correctly on a device? Maybe something like an echo bot with a 5 minute delay or custom delay?