Cannot remove tags from dual-faced cards

I have added some dual-faced cards to my deck. I added them under the tag "for-free-slot". (I categorize cards in my decks by "function", with the above being the category for freeform/fun cards)

Later, I decided to recategorize them under the "mana-source-lands" tag.

However, no matter what I do, I cannot remove the "for-free-slot" tag from those dual-faced cards! I've tried removing the tag from the front face; I've tried removing it from the back face; I've tried removing the tag from the front, then adding it to the back, then removing it from the back.

In all of these cases, I am seemingly able to remove that tag -- but then as soon as I sync Urza to the cloud (eg. "upload", wait for confirmation, then "download" again), the tag I wanted gone returns somehow. (and I've tried syncing using both OneDrive and DropBox, so that's not the problem)

Anyway, the two cards I'm currently having trouble with are: "Bala Ged Sanctuary (180)" and "Khalni Territory (192)"

But I have had this same issue for other cards before. I have ignored it in the past, because I didn't focus on the tags that much -- but in this case it keeps distracting me because it's messing up the card-count quotas I've set for the different functions/categories.

If it would help, I'd be happy to send my collection and/or deck exports to try to help debug it.