I am also experiencing precisely the same problem after installing the offical Xcode 10 release. Having then decided to roll back to Xcode 9.4.1 it seems the problem is persistent and what was working before under 9.4.1 now no longer works either.
Only iPhone SE, 6, 7 and 8 in non plus sizes show in the simulator and only black screens show in all plus sizes, iPad and iPad Pros.
Clearly this was not fixed in Xcode 10 Beta 2.
Is there any updates on how and/ or when a solution to this will be found/ released? I read in a forum post that this was related to Intel 4000 chip set. But my iMac 27" along with many others here is running the ATI Radeon HD 4850 512 MB.
iMac 27"
High Sierra (10.13.6),
Processor: 2,66 GHz Intel Core i5,
Memory: 12 GB 1067 MHz DDR3
Graphics: ATI Radeon HD 4850 512 MB