https://valheimbugs.featureupvote.com/suggestions/158557/attacks-not-registered-on-slopes

And I imagine it crossed many minds that this went from a bad bug (not bed bug) to a terrible one with the Mistlands update.

So: ++

But there are a few mods which supposedly fix this issue.
Tried two recently and both were terrible because they spammed BepInEx's console with some null-vector error every few ms.