Version 1.1.2
I recently completed the Vindland set of Missions including all the mysteries and Wealth, however for some reason it will not show up as completed.

I have tried the following...

• Fast Travel to a point nearby and going back to the location again. The Moose starts by showing as standing (from a distance), when I get close it automatically falls to the ground with no way to interact with it.
• Travel back to England, then return to Vindland same result as above
• Load from a save point and try the first point again
• Close the game completely and restart, load and try the first point again

As part of my process I investigated which Mystery wasn't showing as complete, I was able to confirm that it was this one as it is not showing in the Legendary beasts section as complete, however on the map in Vindland there is an X on the map where it would normally be marked as a mystery.

In every instance it is the same result.

I have attached images below, the first to show the beast dead, the second to show the mystery still open, the last to show the beast not marked as dead.