Unexpected results for luma darken only
This is for GIMP-2.10 and also for GIMP-2.99. In the attached XCF file, there are three layers. The layer masks are set up so that Sample point 1 always samples from the top layer, and Sample point 2 always samples from the middle layer. There is a selection around Sample point 1 just to show visually where it is:
Here's a screenshot showing the above XCF file (on the right), and a duplicate of the XCF file (on the left), along with the layer stack and the Sample points:
Notice that Sample point 1 has a "Y" value of 0.145418, and Sample point 2 has a higher Y value of 0.184187. So if the top layer is set to "Luma darken only" one would expect that all but the square around Sample point 2 would be the color of the top layer. But the only way to make this happen is to right-click on the top layer and change the layer blend mode from "Auto" to "linear", which the only way the XCF file on the left in the screenshot differs from the XCF file on the right.
For the Luma lighten and Luma darken layer blends to make intuitive sense, It seems to me that "Luma" needs to be replaced with "Luminance" and the operations need to always (or rather "by default") work on linear RGB. Otherwise the user will be very puzzled as to why a darker shade of brown is somehow "lighter" than a lighter shade of red.