What are the possible reasons for a piece of a body mesh not rendering in game?

I have a playable character with hooves, where the lower two sections of the Better Bodies mesh have been replaced with custom ones that provide the heel joint and hooves.

This was working fine, until I tweaked something recently in the mesh and texture to reduce the seam between the two. Now the "Right Ankle" section has disappeared, but the "Right Foot" remains visible. Both share the same texture, to the point of using a common set of nodes in the nif for Material etc. That leads me to eject the texture change as the source.

It's completely transparent in-game, rather than showing with a plain black texture. I doubt that it's a normals issue, as I'd be able to see the inside surface in that case (unless they went completely missing?)

I did have an issue with the Right Knee mesh getting duplicated, but they never disappeared, and are now back to normal (pun intended, as the duplicate had flipped ones). All looks good in both Blender and NifSkope.