Sometimes when my pets are exploring, the dark patches don't always go away at once when I approach them. If the darkness is covering an item or an enemy (it doesn't seem to happen on blank squares at all), the patch will occasionally stick around for another couple seconds before disappearing as well. Sometimes I'll have to move my pet again to get it to go.

I've tested the bug with several of my pets over the span of a couple days, and it seems pretty indescriminant between them, though random in terms of when or how much it happens. The screenshot provided shows two squares staying dark, concealing two enemies (I have another screenshot of after they finally cleared up if you need to see that too).

It's more of an annoyance than anything, since I keep accidentally running into enemies thinking it'll be loot, but I can live with it if it's just my computer or browser being iffy. I have a Macbook Pro, running OS Sierra version 10.12.4, and I'm using Firefox 53.0.2.