Keeping battery health, bms board transfer

I run my own repairs business and i offer to keep the battery health after changing the battery by using qianli tag on flex and spot welding the bms board to the new battery. I’ve done this before but on this 12 mini my customer claimed that sometimes when the phone was at a low percentage like 10% it would just drop to 6 or other times it would just jump down a bit. Whilst i was welding there was a little spark that came out but I believe this is usually normal. Im trying to figure what this may be

Have i shorted the battery

Or something else gone wrong

The battery health is 100% though. He also said the phone doesn’t overheat or anything

Is there any way i could resolve this issue