1st Fenumal Scorpion (unique into another random unique) spends the beast but does nothing

The first time this happened recently, I thought I got tremendously (un)lucky and got the same unique back. I hadn't checked the mod ranges to know either way.

However, this just happened again (Hrimnor's Hymn, T5). I again hadn't checked the mod ranges, but the odds of this happening to me twice in short order seems farfetched. I've done like 3 Fenumal Scorpions the entirety of Phrecia.

Seems like it might be the first Fenumal Scorpion per Menagerie instance, or if it's the first Beast you go for upon entering. I had 3 Fenumal Scorpions lined up today. After wasting the 1st, the 2nd one did actually change the unique. The 3rd did as well.