I'm assuming you caught all these manually, because if they are not picked up in a bucket at least once by you, they will be able to despawn. I also do not recommend giving your axolotls access to land, primarily because of their tendency to crawl onto it, wander far off, and not return to the water before they die. I've witnessed this happening first hand with mine, I'm not sure if it is a bug, but if you block off all access to land this should be solved.
I'm assuming you caught all these manually, because if they are not picked up in a bucket at least once by you, they will be able to despawn. I also do not recommend giving your axolotls access to land, primarily because of their tendency to crawl onto it, wander far off, and not return to the water before they die. I've witnessed this happening first hand with mine, I'm not sure if it is a bug, but if you block off all access to land this should be solved.