Scripts in play solo have no concept of client/server. LocalScripts can create things under FE and Scripts can read them, everything runs with the same _G environment, Scripts can change values like LocalTransparencyModifier that don’t normally replicate, LocalScripts can listen for OnServerEvent and vice versa.
These are all limitations of having a single instance. In order to make module scripts work under play solo we’d have to cache the module based on the script’s ClassName, which is super weird when all these other features are not split based on the script’s classname.
At this point it’s way too late to make this change without breaking a ton of games in play solo, but that’s one reason why it won’t work.