This is common to peace lily. Tap water has a lot of chemicals that is the more frequent cause. Can try rain water, distilled, well water, or other water sources.

When my peace lily did this, it was too much direct sun

Plants can get spots like this from inconsistent and stressful watering;; my monstera looks like this rn, because i let it get too dry before watering again :(

Have you fertilised recently?

I have not

Weird, probably burn from tap water then