It is not your responsibility to touch their code in any way. The POS that we integrate with makes changes all the time. Sometimes, these affect our product &

It is not your responsibility to touch their code in any way. The POS that we integrate with makes changes all the time. Sometimes, these affect our product & we have to fix them. It not a single circumstance have we ever asked them to fix the errors, nor would we. I would stop doing any maintenance work for your customers immediately. I don't know if the examples you mentioned are just edge-cases, but to help them, you can just communicate what you have changed, why etc, so at least they are aware before their issues begin.