The way the multires works is it basically builds a hierarchy of subdivided versions of your mesh that you can interpolate between and edits you make in lower subdivisions will propagate to finer subdiv levels. It cant "add" geometry, only subdivide it. You probably just had some janky geometry that got revealed whenever you subdivided it.
The only thing about the multires mod is that it works best when your mesh is in a somewhat "static" state. It builds that hierarchy of subdivisions based on the mesh whenever you subdivided it. If you go in and delete a few verts and add 5 faces then that entire hierarchy is thrown off. Which is whats causing that spiky issue.
You have to "reset" the mesh subdivision in the Multires like this.
You might lose some work but that's why the Multires works best with a "static" mesh. If you're doing alot of big changes, snakehook, grab tool etc then you should be sticking to just using voxel remesh. Then once you get to a point when you're trying to "polish" and you cant get the crappy remesh geo to look good, that's when you QuadRemesh it, Multires, Shrinkwrap to the original, and then you add your details and clean your mesh up.