Hey guys,
There is a bug in 4.12 related to transparency which is being investigated by Epic currently (bug report ID UE-31589), here is the bug report:
[MENTION=4894][/MENTION] actually just replied an hour ago saying the appears to be resolved in version 4.12.4, so update your engine and see if that fixes it.
If any of you are already on 4.12.4 and still have this, please reply to the AnswerHub thread I linked above with an image and description so they can investigate further.
Thanks!