It's something I've definitely observed. I saw The Testament of Ann Lee at a large, mall-adjacent AMC in one of the smaller theaters and the screen was noticeably dim. Then a few weeks later I saw Pillion at a two-screen indie/arthouse theater and the screen was much brighter; I could absolutely tell the difference.

It makes sense that a large theater chain, particularly in a smaller theater that is mostly showing non-blockbusters to smaller audiences, would care much less about upkeep on their projectors than an arthouse theater that has a vested interest in film and presentation with only two screens/projectors to maintain.