• Your Rocket.Chat Experimental app version: 4.2.2.999999
• Your Rocket.Chat server version: 2.5.0-develop open.rocket.chat
• Device (or Simulator) you're running with: Android P

It is a minor improvement. As the gif shows, the Pin message feature is not working on direct message room. I checked the web client, and it doesn't provide the Pin message feature on direct message so on mobile client also, the option should be hidden for Direct message rooms.