From discussion with Neal, the expected behavior is that resources should not be embedded into secondary assemblies:

• refout: main assembly has resources, ref assembly doesn't
• refonly: ref assembly has resources

@jonathanpeppers reported that updating resources causes the ref assembly to change.

A quick look at the implementation and tests make me unsure about the current behavior, so I'll investigate further.

Ref assembly spec: https://github.com/dotnet/roslyn/blob/master/docs/features/refout.md
Ref assembly main PR: #19417