Problem with Steam Cloud save. I would turn it off.

I play on two different machines - I wonder if that's related...

Never use Steam Cloud on any game, ever. It is a buggy mess and Valve doesn't bother to fix its many issues because customers direct their anger at each games developers instead of Valve, despite it being a problem with Steam.
Any slight hiccup can cause you to lose sinc and loose hours worth of progress on multiple games. As well with Valheim (among other games) if someone logs onto the same computer under the same Windows profile and then onto Steam, it will replace whatever save data was previously there with the new persons data.