The multiply tool does not seem to work properly on CMYK drawings. Instead of making the colours darker, they get lighter. Would appreciate if this got fixed in the future

Multiply is a mathematical function. You can fake it working on CMYK by inverting the multiplication, but that is, actually, wrong. The way Krita works is actuall correct.

In any case, you should not draw or paint or create a layered image in CMYK: that is something to do in the last stage, after merging down, when you need to remove impurities and problems from the final image.

This is not something to argue about; it's just basic maths and color management.