Every mic that I have built sounds slightly different after a few days of use.
I can absolutely confirm this is true. I do measurements daily, and i have to re-calibrate the setup before every measurement, otherwise i get all kinds of artifacts.
However it is almost always due to capsule/environment changes. Humidity, air pressure, temperature and even the objects around. If i'm within a meter from the mic my body position will affect the response. Position of the mic stand and how it's angled... All of these make very audible and measureable changes in mic performance. Several db of difference in very sensitive areas.
I've been struggling for the longest time with multi pattern mics because of remaining charges on capsules even when pattern is changed. Sometimes there is no path for the capsule to discharge depending on topology. There is good chance everyone has been using a wide (or hyper) cardioid while they were thinking they were using cardioid. And suddenly the day after the mic sounds different.
One of the reasons i never use multi pattern LDCs. At least not the conventional ones. The other being you get mediocre responses with anything other than cardioid, and all sorts of phasing issues where "electrical" pattern fights the "mechanical".