Villagers in my world are disappearing

My villagers who disappeared was trapped in a place with good lighting and they were tagged with nametags.

 

The path I took before he disappeared was from my house (which is next to the villagers' place) to the gold farm which is basically about 100 blocks away. I stayed there a whole day and went back to my house to sleep and the next day I went to check on the villagers and one of them disappeared, just the one in a chunk border.

 

Details:

• World singleplayer

• The world is weighing 57mb

• Rendering distance: 14

• I play on an android device with 8gb ram