Essentially, I might be dumb or something, but when painting over a mask, regardless of whether I am doing it in 3d or 2d view, multiple faces that have absolutely no relation to each other get painted over. They have no UV space overlaps and they're not even close to each other. Example on video: https://youtu.be/EETRMgFpy7Q
The models are made in blender and also exported from blender. I tested this and it was a problem both on a 2019. january version and on a new 2021. november version. I am using Windows 10, build 19044.1826.
Perhaps this has to do with SP's file settings? Or Blender's export settings? I have no idea and I've encountered this before. Thanks for reading.
It appears I've found how to replicate it on my end. When using the "dirt splash" brush with size of at least 20 (this doesn't appear to be a problem with smaller brush size), it paints over multiple faces. When using the brush "basic hard", it doesn't happen even if I change the brush size. Demonstration on video: https://youtu.be/7WYjWEMpLSg
The log: (I don't know whether this is the log you were looking for, but this is all I found and this is all it displayed).
I have a growing suspicion on me that it has to do with how the project settings or the UV map are set up, but this is definitely puzzling for me. Perhaps the solution is easy and not even a bug.