GIMP Version: 2.10.10
Operating System: Windows
OS Version: 10 x64
GIMP Experience: Experienced User

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Having a really debilitating problem with layer masks since updating to 2.10.10. Everything works fine until I hit "Apply Layer Mask", which of course is supposed to multiply the mask onto the layer's alpha channel, except that now it's multiplying onto the color channels as well (like you would do for premultiplied alpha), thus darkening everything quite drastically. I'm unsure if this is a bug or some premultiplied alpha feature that's been added recently and that I'm just not finding the settings for.
Any help would be greatly appreciated, as GIMP is basically unusable for me now and I'd hate to have to roll back to a previous version because of this.

Edit: 2.10.8 does not have this problem, so this is definitely a 2.10.10 thing.

I'm not seeing that problem here in Gimp 2.10.10, it works as it should. Perhaps you have the layer mask set for channels instead of alpha?

Same here: what I did:

- Create the FULLRGB gradient (lots of bright colors)
- Dupliccate the layer
- Add a mask to the top, fill with black-to white gradient (no visual change as expected to the whole result)
- Apply the mask (this should darken some areas, especially around middle B-W gradient, but it doesn't).