To me this looks different than MCPE-21856.
I can confirm what the issue starter says. If we play together (LAN, my pc) in our bedrock edition (Windows 10 v. 1.8.0) no monsters are spawming. As soon as the other leaves the world, monsters are spawning immediately. And plenty.
We have the same thing in a slime farm. It did work properly before and nothing is changed in the slime farm.
When you enter the farm for the first time, there are some slimes (1 or 2), but new ones don't spawn. No matter what the moon is like (not even during full moon). If you return another day it is the same.
If you leave, go to peaceful and back, come back during the moon (=night) there are 1 or 2 slimes again.
Issue MCPE-31275 was closed as a duplicate of this issue. But it seems a different problem. (commented this to this issue also).
If we play together (LAN, my pc) in our bedrock edition (Windows 10 v. 1.8.0) no monsters are spawning. As soon as the other player leaves the world, monsters are spawning immediately. And plenty.