5W Charger not working with my iPhone 14 Pro Max but my 20W does.

Hi so I have a lot of chargers at home and use my 5W in the living room due to it being used as an extra charger. Anyways lately I've noticed it doesn't charge my devices anymore. Like at all. It used to, all be it slow, but at least it gave some percent of power.

If I use my 20W charger it works as intended. Thank god.

I've been to the Genius Bar locally and they ran some test and can find no hardware nor software problems, since we reset the phone without a backup, yet the problem persist. While they did their best ultimately we landed on just keep using 20W chargers. But I'm still curious as to why, and if more people have this issue.

To summarize, my iPhone 14 Pro Max won't charge at all with 5W chargers. Why?