Artifacts are occuring after some tweaks in edit mode

You want to be very careful about making edit mode changes on a multi-res based sculpt. You just need to be very mindful that it's a modifier and each change in edit mode propagates up the stack of subdivision levels. We use this to our advantage with the mouth-open-eyes-closed shape key. But it can really go badly if we start making wireframe changes.

Basically moving verts around is pretty safe though limited to the base level of geometry. As in we can't move a problematic vert on level 3 for example. Keep in mind that operations like merging verts and knife-cutting new edges - these change the mesh structure (wireframe). This means multires has to figure out what to do with that new data through the subdivision levels and usually the result is funkiness around the area changed. Frankly I've always been shocked that multires doesn't explode with mesh structure changes - for that I'm grateful and fixing funkiness is a small price to pay.

I used smooth brush to manage the artifacts but now I lose the pore details. How can I maintain all the details while changing the vertices.

By this point we really need to have committed to our base mesh structure during the retopology stage. I mean committed in a way that we don't make any further mesh changes. Changing mesh structure is heinously risky once you're in the millions of polygons and have pore detail sculpted. With any change you can expect cleanup if not a broken sculpture.