Silly bug? Please heelp

My apologies for the late reply jjonaskiste - I hope this isn't too little, too late. I've seen several issues that seem to be related to multires data which appears to be prone to bugs, including baking artifacts.  In such cases, my recommendation is to “reset” the multires data:

Method 1: Apply the multires data at the highest level. This will slow down your scene tremendously so be patient and methodical. Once applied, add a fresh multires modifier and click “Rebuild Subdivisions” under the Generate tab of the modifier UI.

This will create all your lower levels of subdivision back down to the base (when it can’t mathematically unsubdivide any more). This will take a long time given the high polycount. I’ve seen it take 10, 30, 60+ minutes. But once it finishes try baking again as usual. Hopefully this “refresh” solves it.

Method 1b: If the above doesn’t work, sometimes I will export the mesh and re-import it as an additional measure of “refreshing”. So it’s the same process as above except I will export as an .OBJ file after applying the original multires modifier. I.e. export your highest resolution sculpt. Of course be patient with this as well. Once it’s exported, re-import it into your scene and continue with the fresh multires + rebuild subdiv process.