BTW, before investigating in creative mode, I tried temporarily unsubscribing it, then resubscribing to check if it was some sort of weird corruption to my local copy of the blueprint file. But the fresh download didn't fix things.

@Fuji This has been one of my favorite base designs since I found it a couple of years ago!

However, in a new (vanilla) survival play-through, it was marked as having a forbidden block and that prevented it from using it. I switched over to a creative game and used the prefabinfo command and the offending block type was listed as "391 HullFull (Steel Block)". It turns out there wasn't just one of these, instead there were around ~80 of the steel blocks that had health of 50/50 (regardless of shape, including full cubes), and recovered as "Steel Block S". So, in creative, I replaced all the steel blocks that appeared as "50/50" of the structural blocks in the base with "Steel Block L" types and that solved the issue.

I know you've probably moved on from vanilla to RE2 exclusively like many (if not most) of the talented designers. However, I'm letting you know about what I encountered, and also to inform anyone else who encounters this issue about the solution I found.