Anyone have any idea why the thief in this situation (https://imgur.com/a/6BN7RL0) doesn't move on his turn, even though he has an open path to his goal?

I noticed a couple irregularities earlier in the same run with this same thief (he starts in the right side). For example, when he couldn't move forwards because he was blocked, he actually moved backwards (first time I see a thief retreat). I thought he was going to go all the way around the mountain to the left side, but immediately proceeded to get stuck in another place.

I know this is ultimately unimportant, you just kill the thieves and that's it, but given the relatively complex movement logic for opponents and autobattle I expected better from these guys.

Edit: A few more tests, Selkie and Azura can be anywhere on the map and the thief won't move as long as Peri and Vero's spaces are occupied. Also added image to post body (couldn't on mobile).

https://preview.redd.it/lbuo3ycgzwy61.jpg?width=1080&format=pjpg&auto=webp&s=2c599375496c9a4db39d3b6cb194ff52798f86d8

Edit 2: This album for a couple cases more with the same thief. Depending on how my units are positioned, he chooses a different tile to stay in and not move.