Thank you for sending that reproduction project. While looking at the settings, I now see what I was missing. Firstly, I’ve never used Ambient Cubemaps so I wasn’t aware that the intensity needed to be activated for it to actually do anything. Secondly, I hadn’t be setting one of them to Unbound, so although they were overlapping in my scenario, the issue wasn’t reproducing.
After looking into it further, it seems that there was already a bug in for this particular freeze but it was originally thought to only be a conversion issue. Since we have reproduced it from a fresh project, I’ve updated that bug with the new information. For reference, the bug’s number is UE-31945.
Unfortunately I can’t think of a workaround at the moment other than simply going back to 4.11 as this seems to work correctly prior to 4.12. I’ll be sure to update you when any changes are made to the bug report.