sun burn

I agree with sun burn, but it also leads me to believe it may be red spider mite damage - check the under-side of the leaves - any tiny red specks that move when touched? The dark spots have something to do with water - always soak early in day so surface is dry by evening - also check for good drainage and never leave water sitting around the roots. You may want to consider a spray of fungicide and insecticide a few times a year. Disease is spread very easily with water - you may also want to quaranteen these plants from your others until the mystery is solved

Thank you both for the comments. :) sunburn actually makes sense because I actually moved them all to a new spot maybe three weeks ago that is less shady. It's still shady, but I guess less shady compared to the old spot (they were 100% indirect light before). I moved them all to a new spot in between the old and new to fix the problem hopefully

I don't see any spider mites, but I'll keep on the lookout. I spray them with Bayer Natria insect, mite and disease formula (I think it is called that) every so often because last spring one of my phals got mealy bug infested and kept dropping its blooms as soon as they opened.