I have noticed this recently that the community section of the app does not display the virtual keyboard when you try replying or creating a new topic, therefore you need to you a PC if you’d like to comment / contribute.
This bug must be introduced recently as I have used the forums on my phone in the past.
The virtual keyboard works fine in other parts of the app (like chat support, adding a note to a transaction etc)
My phone is: Xiaomi Redmi 4x
Android version: 7.1.2