Sorry for the late reply. I tried your suggestion, purged the cache, reset a copy of the RAW file to original raw settings and managed to get the spot removal tool to work successfully. Afterwards, I applied other effects.
I went back to edit the same area again with the spot removal tool and had no issues.
I'm glad there's a way around this glitch, but I really wish Adobe would get on top of squashing bugs like this. With the new awkward UI, it hasn't been a fun time editing photos recently -_-