You reminded me; I am indeed using a modified version of Krita from a user in the community, from this link: Krita with clipping (test build) - #83 by _LimaoComSal . Clipping masks really help me reduce the creation of many unnecessary layer folders, and I’m also using two types simultaneously.
I downloaded the original 5.3.1 version from the official website again, specified a completely new resource directory to regenerate all configurations and release new resource files, and then tried your plugin again, but the result is the same as in the video above: the entire layer is still filled with that grayscale value.
I also tried Direct3D 11 and OpenGL in the display options, and the result was the same.