TPA3118 + LN-BT02 clipping?

So I'm building bluetooth speaker box. Just one speaker.
I take my Bluetooth module, use 1k resistors to sum the stereo into mono. Feed that into the single amp channel.
All works great.
Till I turn up the volume past 75% and it goes blahhhh (so to speak).

TPA3118 60-watt mono board:
DC 12V-24V TPA3118 BTL 60W Mono Digital Audio Power Amplifier Board Module M374

LN-BT02 Bluetooth Audio Receiver Module:

Here's the scenario just now.
I use my cellphone's bluetooth to send a 200hz test tone to the BT module. Which gives the summed signal to the mono amp. Which feeds a 3.5" NS3 full range speaker.
Sounds great.... until I get to about 3/4 volume when suddenly the tone goes weird, higher pitched sounding. An extremely audible difference in pitch.

So I put my voltmeter on it and at low/mid volume the 12.6V LiPo battery pack (freshly charged) will be at ~12.5V all the way until I get to that barrier where the high pitched sound takes over. And suddenly the battery pack shows ~5V.

Is this a symptom of signal/amplifier clipping?
What can I do to resolve this?

I don't have any AUX wired to the module so the only signal I can send it is from my cellphone.
Hmm, I may try my computer too.
Hmm, I tested it again and don't get the 5V issue anymore.
Tested it on my PC's bluetooth and had the same issue with the speaker going poop on me.
Then hooked the speaker up to my LaPai 2025 or whatever it is and had the same issue.
Must be the limit of the speaker I guess.
Tried two different identical speakers so I doubt it's DOA.

Don't have the issue with a 4" coaxial that has higher sensitivity.