Killing Rares on Maps is not contributing to Gambling Restock?

Wanting to gamble on Act 3 normal for a pair of gloves, Alva needs to restock through killing rares. Being in Endgame and killing lots of rares - that does not contribute to Alva's stock.

Is this a bug or intended?

Also, Alva in Endgame has infinite stock. But this is useless if a player needs a low-level unique for his build.

Any ideas?

The stock is replenished by defeating unique enemies, not rare ones.