Unfortunately I don't know what can be the cause, but it seems something is not right when flashing on source built custom Roms.I'm using Pixel Experience June patch. Even with a completely stock PE boot image (no magisk), flashing 5.1.1_S causes the bootloop immediately.
The reports started when magisk 25 dropped though even with zips that previously worked. So it might not be the kernel, but rather down to magisk and/or something the custom roms changed.