Lightroom Mask tool is leaving huge black "sponge"-looking marks on my edits.

Is anyone else having this issue, and is there a workaround? My raw images are fine; no upload issues. However, I use a transition-lens brush on applicable glasses for clients. When I object-select and mask after the brush is applied, it leaves this hideous and pixelated black sponge mark over my images, intermittently disappearing and reappearing, making it difficult to make adjustments.