So an HDA gets the mesh data from Unreal 5.3, including collision. In Houdini 20 I separate the collision prims from the rest, process the rest then merge the collision prims back before the output node. When the mesh arrives back to Unreal it does have colliders but they are a bunch of unrecognizable triangles stretched to infinity.

As a debug step I instantly passed the input data to the output and the result is the same. The incoming mesh has proper UCX collisions.

Changing the group names from "collision_geo_simple_ucx0", "collision_geo_simple_ucx1" and so on to "collision_geo_simple_0", "collision_geo_simple_1, etc didn't help.

Is there an example HDA that demonstrably produces proper collisions? A 100% certified reference would help a lot.

Just ran into this issue myself today. I found a workaround, and since this was one of the few posts about it that I could find online, I'll just share it here:
I found that if I renamed the collision group from "collision_geo_simple_ucx0" to "collision_geo_ucx0" it worked perfectly.
According to the Unreal Houdini Engine documentation page, this format invokes the "Complex_Collisions" convex collider geometry generation (invisible).
More info can be found here [www.sidefx.com].