Good news!
I can confirm that this issue has been fixed in the branch, absolutely no issues with it anymore.
The changes made occurred before the recent branching of 4.13 (probably should have built that one instead), but I compared the shaders on Git and they are the same so it will be in 4.13.
Looks like the change was made for the Metal API, causing a regression from 4.11. The new code includes a #IF COMPILER_METAL check for the affected line/
Thank you guys! Sorry for my mini rant , was just a bit surprised by it. =)
EDIT: Should probably update UE-30488 when you guys have a chance!
For reference here is a link to CL #3063329 where the change occurred.
Thanks again.