Using 2 remote parts, file not being copied from stage to prime

I'm trying to use 2 remote parts simultaneously in this simplified YAML:

name: dual-remote-parts-bug
version: 0.1
summary: Bug
description: Bug
confinement: strict
grade: devel

apps:
  dual-remote-parts-bug:
    command: ls $SNAP/lib

parts:
  bug:
    plugin: nil
    after: [desktop-gtk2, xwayland-kiosk-helper]

The second remote part "xwayland-kiosk-helper" is defined here:
https://github.com/gerboland/xwayland-kiosk-helper/
It creates a file lib/libxwayland-preload.so which I see is created in the "stage" directory, but it is not copied into the "prime" directory for some reason. But the bin/xwayland-kiosk-launch file is copied?!

There is no "prime" specification in the remote snap, so shouldn't everything that is staged be copied into the prime?