"Do not disturb" in dtek50 not working - android or bb bug?

Since the latest system update about a week ago (probably the android security patch, May 1, 2018), when I put the phone in "do not disturb" (alarms only) mode with "until you turn this off", I can see the icon at the top of the screen - something happens and the phone goes back to ringing as usual by itself.

I don't know what this "something" is, and it's most annoying when the phone suddenly begins to buzz and ring in the middle of the night.

I rummaged through every possible setting, looked for apps with admin privileges - and couldn't find anything.

Could anyone (pleeeeeze) help me to get a sound night's sleep?

Thanks!