The registration hook is to register modules. Don't use it to add entries to $wgResourceModules.
This was never supported and happened to work since the hook was run in ResourceLoader.php exactly one line //before// the modules from wgResourceModules are registered.
This is changing in MediaWiki 1.26 and therefore this code must too.
Other extensions always used `register()` when inside the ResourceLoaderRegisterModules hook, that's what the hook is for.
To register modules in `$wgResourceModules`, either add them statically (in extension.json or the PHP entry point). Or, to add them dynamically, do so from a setup hook (e.g. wgExtensionFunctions).
The main culprit is `MobileFrontendHooks::registerMobileLoggingSchemasModule` which is re-used in multiple code pieces instead of explicitly doing it from one place.