I have a law of nature for this:

The Spanish proverb “en casa de herrero, cuchillo de palo” (literally: "in the blacksmith's house, a wooden knife") means that the things you would most expect someone to have or fix are often missing or neglected in their own home or life

In UK English we have "the cobbler's children have no shoes", which seems similar.

I always thought it was a statement about poverty, but online sources say it's about not applying one's expertise to one's own situation. I'm not sure I agree with that interpretation.