Oh, that's unexpected! Could you check if the achievement is still greyed out and whether the stats screen says "Won!" for those runs? I'll see if I can reproduce it.
I was able to do a run where I got the achievement! Could you take a screenshot of the stats screen for those runs or submit feedback in-game? Either will help debug what happened.
If you'd like, there's a Discord server where it might be easier to hash this out: https://dsc.gg/bowtieguild
Alrighty! I took a look at the debug data you sent. Not sure how it happened, but it seems somehow that achievement value became null. Here are some steps to get it back in a good state:
1. Go to %appdata%\Godot\app_userdata\Decks Out
2. Open metagame.save in a text editor
3. Change the achievements value for "OtterFavorite" to 0