TranslationService uses origin since MAGNOLIA-6234. Origin doesn't support same name files, so only the first translation file is loaded. This affect modules which have mgnl-i18n/messages_en.properties files, e.g. /magnolia_ui/magnolia-ui-admincentral/src/main/resources/mgnl-i18n/messages_en.properties.
The solution is to move the files to the new moduleName/i18n location or rename the file.

Go trough all modules and look for mgnl-i18n/messages_en.properties file and check if more modules are affected.