Totally read you and totally agree. I got a video of it happening on my phone, so the shop couldn't argue. Maybe try that? End of the day, a fault is a fault, insist on your consumer rights. In your case it does sound like a bad connection in the battery or cable.
I get the same issue on my 2022 Comp Carbon. I've had it into the dealers, who contacted Spesh, who sent a new battery connector cable. This helped a bit, but the issue still occurs. Basically you have to fully and firmly push home the plug and it'll then work (on mine at least).
Next time it's...