Whenever we name a mob, lets say a sheep and leave it in a pen. It still manages to de spawn. Same for axolotls, the only reason we know these mobs aren’t dying off is due to the location of where they are kept. I keep my axolotls in my house in an underwater oasis so they don’t climb onto the surface and die yet they still keep de-spawning. There’s nothing we are doing that could cause this. We’re just playing Minecraft regularly. My phone is an IPHONE XR, my sisters is an IPhone 6 and my brother plays on an Xbox. I wish I had more information to help but that’s all I have.