Aha! I may have an answer. I talked to my friend Karo and he says you almost certainly didn't cause offense.
It appears we've been thinking about it backwards - we all assumed that you had inadvertently insulted them by going against some manner of cultural guideline. The truth may well be that he was worried he would offend you by not responding correctly. For example, did you expect a gift back or a card? So when the son said "she's bought us a plant" what he was actually thinking was "tell me what I'm supposed to do." As it was around Christmas time he may well have assumed there would be lots of traditions around gift giving and that if he got it wrong he would upset you.
Basically it seems like a cultural misunderstanding, with both parties worried they have insulted the other.
This would fit with things returning to normal now - if you stayed away they may have presumed that they had offended you culturally. As I've said, it's an innocent misunderstanding. You may want to explain that you took no offense, or it may well be settled now. Either way, you weren't in the wrong.