I'd expect a partial short to cause a significant impedance difference, leading to a noticeable difference in volume.
I doubt any normal QC check would pick that up.
Thomann are poor at understanding technical faults. My brother had a HB with mis-wired controls and they asked for videos. Although the strange control behaviour was obvious in his video(s!) they just saw both PUPs worked and wouldn't accept there was an issue.
So... he just said "I've decided I don't like it", returned it and then ordered another one. No questions asked. 🤷♂️ (Same thing works at Ikea apparently "this mattressdoesn't fit teh bed you said it would!" - "nothing wrong with it, we can't change it!" -"I've decided I don't like this mattress" - "OK, I'll process a refund."
In contrast, I got a Joyo amp with a euro plug and no adaptor, emailed them, and they sent one out next day.