Here’s my answerhub conversation with myself: Level Streaming failing on 4.15 - MAC & iOS packaged, Cooked PC - UE4 AnswerHub
The skinny is - calling OnMountPak or FPakPlatformFile::Mount prevents further loading from Content (such as level streaming).
It only fails on Mac and iOS packaged (MAC PIE works fine).
It continues to work on PC, PIE and packaged…
Given I’m targeting an imminent release on iOS this hoses me completely. I may need to work out how to return to 4.14.