This is an already known issue:
when you select the world, click on "modify" > "optimize world" > "create a backup and optimize"
wait until the process has ended and open the world, the maps should update back to nomal
Happened to me too in a world I had since 1.12: 7 wolves, none of them standed up, I had to kill all of them :'(
The only explanation I can figure out is that there is a problem when you update a world to a newer version and for some reason it happens
Ludovico Fusco
This is an already known issue:
when you select the world, click on "modify" > "optimize world" > "create a backup and optimize"
wait until the process has ended and open the world, the maps should update back to nomal