I also have a similar issue. Issue only appear for me when in an amp charger, nothing shows on screen indicating it is receiving any traffic. LED on charger also does not indicate traffic. When issue occurs, it acts normal when taken out of the charger. I get no audio from the pager speaker or charger speaker, the audio comes out of PA system connected to the green output on the charger.
Sometimes a subsequent tone resets it, other times no amount of reboots, pulling battery, or reprogramming fixes it - I have to delete all data, reset to factory, reflash firmware, then reprogram codeplug.
Originally, I thought it was failing to reset after a page. Then, it started occurring without a page.
For the longest time, support blamed me for failing to understand how it should work and my bad programming. Once I was able to capture video of the issue occurring, they took it seriously. I've been sending diagnostic logs to support and I'm told they're looking into it. I have a large fleet, and operating on several firmware versions, they all act the same. Test firmware fixes have also failed to fix it.
I'm starting to look into other alerting products because this, as well as missed pages, has became a major issue.