Nametagged Mobs Disappeared off my Realm

I've had some mobs disappear off my realm. I had 6 crabs (2 of each color) and I believe 4 penguins (2 of both breeds) in a closed exhibit in my zoo on my realm. They were all name tagged and there's no way a person could've killed them/ gotten rid of them, they're simply gone. I just noticed today so I'm not sure when they disappeared but I would just like to get them back because it took a while to collect all of them. Also I have a lot of other mobs in my zoo for a while now and the crabs and penguins were the only ones to disappear, possibly a glitch?