I do not believe that this is a duplicate of MC-208598 because there is a layer of air blocks above the surface of the water and then glass blocks above that in my enclosure, so there is no place for the babies to suffocate as described in that report. 

Baby axolotls keep disappearing on me when I walk away from the enclosure I have them in.  I was closely monitoring another baby axolotl to see how the previous one had died or disappeared, and it behaved as expected for 10 to 15 minutes, at which point I walked outside my base to quickly check something a few chunks away. When I came back, both baby axolotls that I had bred were gone from the enclosure. 

Reproduce

• Breed two axolotls that are persistent (i.e. name tag/bucket etc.)
• Have the baby spawn
• Go far away to unload the chunk
• Go back to the axolotls
  Notice the baby is gone