Custom fields not saved for new users registering via mobile app


I have a select box as custom user field in administration → registration which works quite well when users register a new account via web interface. However using a mobile app (original app) the field is also (required) while registering but after that it is NOT saved in user data.
Any hint appreciated!

Server Setup Information

  • Version of Rocket.Chat Server: both 6.3.10 and 6.4.x
  • Operating System: Debian 11
  • Deployment Method: docker
  • Number of Running Instances: 1
  • NodeJS Version: 14.21.3
  • MongoDB Version: 5.0.18
  • Proxy: nginx