I have the same problem, as described above, in 4.18.2
I only use Blueprints, I do not use the CCG toolkit.
I’ve had no success at all recovering the project by deleting & restoring the involve structs and rewiring all references to them: the project files are just pooched. I found it way easier to go back to my last backup copy and treat nested structs as unalterable once compiled.
I see the issue tracker labels the bug as “Not Fix”, but also as “Resolved” and apparently scheduled for release in 4.19. I find that quite confusing.
If it’s not going to be fixed, Epic should at least revise their documentation.
If it is to be fixed and Epic wants, I can provide my Blueprints