Okay, it’s a good news - bad news situation.
The good news is that breaking the mesh apart worked… On the second attempt!
Bad news:
The naming convention appears to need to be (in my example)
UCX_Pin_01
UCX_Pin_02
Doing this works! Hurrah!
Except, when you split something in Blender it is automatically renamed:
UCX_Pin.001
UCX_Pin.002
etc.
So, everything will need to be renamed by hand to change Pin.001 to Pin_001. This isn’t such a problem with this test, when it can be split easily into two pieces, but if I do a long curving wall, then every few paces will need a new UCX_ mesh, and every piece will need to be renamed by hand. The renaming-by-hand bit is time-consuming and annoying… But if that’s what needs to be done then that’s life, it just seems like this must be such a common thing that it doesn’t feel right!
Is this a bug for 4.18 and 4.19? Surely a long concave curve on a corridor wall or on a race track doesn’t need to be made with a new collision UCX_ piece every few meters?
I want to also say ‘thanks’ for the replies so far! We’ve got to the point where I have a functional fix, it just seems like the engine might have a deeper problem here - especially if, as @ThompsonN13 says, a solid with a concave part could have been imported correctly in version 4.17 (although I’ve had a quick try in 4.17 and I can’t get it working).
So… Is there a bug here, or is Unreal really this bad at dealing with concave UCX_ meshes?