I have three tamed cats on a single player hardcore world. After taming two, I fed them Salmon and they got the hearts over their heads.
I expected them to run towards each other and after a few seconds produce a baby kitten.

Instead the cats got hearts over their heads and then ignored each other. They didn't run towards each other and they didn't create any kittens. I doubled checked by making sure they were basically standing (Not Sitting) next to each other and fed them again and they still didn't care at all. 

Steps to Reproduce:
1. Open a 1.16.1 Hardcore single player world. (I don't know if I created this world before or after 1.16 came out. It might have been 1.15.2 but I don't think so/remember)
2. Tame two Cats
3. Try to breed them.