Strider keeps disappearing into thin air

I had a couple pet Striders in the Nether. I built a nether themed house in the overworld and wanted to bring them home. I managed to get three there a couple weeks ago, and it was fine. But I've lost two out of the three in the past couple of days. I brought one of them to the nether, and when I pushed him through the portal to come back, he was nowhere to be found. I checked every single portal in the overworld and even checked the nether again, and he was absolutely nowhere I could find. I just went to the nether once again, and needed a strider so I could get across the lava pool, and when I came to bring him back, he disappeared again! I was really upset about that, and not sure it's only giving me problems now instead of when I first brought them back to the overworld and used to have no problem going back and forth. Hopefully you guys can sort this out quickly.