I remember when the battery on mine went out this last time, it was pretty strange behavior. When it wouldn't start, i got this odd pulsing out of the aftermarket sub, was actually audible in the speaker, never had issues with the sub ever. Voltage was too low, starter wouldn't crank, then "womp womp womp...like 7-8 times" then stop. Once I got it jumped it would start fine, no oddity. But let the bat drain for a few hours, same oddity. Got that battery replaced the next day, couldn't be stranded.
Your issue is a little different, but similarly bizarre. Ideally it's just a bad battery and when they test it they can swap it for you.
Iím hoping for something of the sort, as my battery is still under warranty. I just got a new one about 8-9 months ago and had it tested 4-5 months ago. Everything checked out then, so who knows