I have problems sometimes with the sprint/snap key getting stuck in the held down mode. Wriggling a few keys together normally frees it up (eventually), otherwise a quit and restart.
However, this doesn't mean it's Valheim specifically. It could be the keyboard* itself, or a rare problem in the Unity Engine I once read about somewhere.
*For instance, in my case, did the problem start after I updated the Keyboard firmware, or when I changed the polling rate from default to 1000Hz - can't remember?

Originally posted by A Dismayed Cup:

Walls can still be snapped to their foundations, and beams to some walls, but some things can't be snapped. It's very confusing. Anyone else experienced this?

But this seems to be related to the quirkiness of the system.

Example: Had a nearby core wood structure holding distillery barrels preventing me from embedding a stone slab into the ground, but lowering the ground more than usually needed allowed me better access to snap grid of the floor to sink it in.
Using alternative building parts can also help with problem solving and coming at it from a different direction - i.e. sitting down to get underneath something, scaffolding....