Happening to me on split screen and on another xbox with the same account (when connecting to the host), another account can connect via windows 10 without any issues.
Any known temporary fix for this.
Update 2 : Confirm issue persists with 1.5 build.
Can confirm in the last build (final) 1.2 none beta version this happens (Xbox One Bedrock final release version).
Appears that if all players stay in the same area everything is good (tested several nights).
If players start moving large distances away from each other, the mobs spawn less frequently, as if they are being spread out on the areas the players are moving to and from.
And if you go a LONG way from each other and keep moving around, then the mobs get very very sparse.
As if its hitting a max mob spawn count, over an area that is to large.
Hope this helps.
With the new Preview build and the New Windows 10 Preview build (both latest) it is now working fine.
Using a preview build of the Windows 10 operating system DOES NOT work with the 1.2.0.22 build.
https://bugs.mojang.com/browse/MCPE-32765
see this one, seems this is a duplicated bug.