because that's one of the habits jfm got a lot of shit about, in-story--he lets wei wuxian do exactly as he pleases.
he rushes to cover his ass when he gets into trouble. pays the tab for his shoplifting. he lets him off ongoing punishments, on the implicit basis that yu ziyuan can't be trusted to assign them fairly, so as someone who outranks her he can dismiss her will as wwx can't.
(he can't take away her will, or her authority, without becoming an abusive husband, and i will always wonder why wei wuxian canonically doesn't beef with yzy in front of jfm, though he's perfectly willing to pick a fight in the master's absence. possibly very much as yzy cites jfm's absence as cause to deprioritize that he wouldn't tolerate what she's doing to wwx during wlj's invasion.
what is he avoiding? is it just courtesy, to an authority figure he actually respects? consideration for jfm's feelings? not wanting to disappoint him? it can't just be curating his reputation, because that would be ooc. is he more afraid that jfm won't take his side, or that he will, and then for the first time the problems in their marriage really will be his personal fault?
i think it might be the last one because not wanting to let yzy be right about him does crop up as a motive elsewhere imo. and i like it because it resembles his motives for lying to jiang cheng later in interesting ways.)
this guy took this kid home and housed him in his son's room, instead of with the other disciples, not caring who judged or how jiang cheng or yu ziyuan felt about it. (or more accurately, completely misunderstanding how jiang cheng would feel about it, since his goal was for them to be best friends.) he got rid of the puppies that triggered his trauma without a second thought.
in the only all-family scene the jiangs get (because jyl leaves before yzy comes in during the bedroom scene) the major argument is over whether wei wuxian should be forced to go to qishan with jiang cheng, who is being compelled by his parents due to threats from the wen, or should be given a choice whether to go or not.
yzy wants him to be compelled because that would be more 'fair' to jiang cheng and make a point that wwx is beneath him and less valuable; jfm wants to offer him the choice because he doesn't agree with any of those premises and thinks where you can give someone a choice, you should. jfm wins.
that's the pattern: he prioritizes wei wuxian's happiness and autonomy. even at the expense of his reputation and the comfort of other members of the family, to whom he owes more. he goes as far as he ought for wwx's sake and then a bit further.
and he never suggests there's any debt. yu ziyuan says that. they are at odds. they do not agree. there is not a united 'jiang family' position on this issue.