Perhaps a case at support@mikrotik.com. The path and layer-dir= is about the only knobs I know to try here… Make sure to generate a new supout.rif with the current state and include that too.
Sometime RouterOS has trouble processing the OCI metadata in these “multi-platform” images – which has been fixed at various points – but since you’re using a valid tag… it really should work. But looks like it isn’t.
And, yeah, mounts does want or will add a / slash — why it’s so confusing with the /