Mailbox installed as app on Android: How to copy paste?

Hi

I'm pretty new to Mailbox as I'm shifting away from Google. Until now I'm pretty excited, but I have a question:

My Fairphone 6 is running the e/OS version of open source Android (degooglified Android). I use Vivaldi as browser and I have logged into Mailbox and installed it as an app.

The app is nice and clean - great. However, for some strange reason the default way to copy paste on Android seems to be disabled when I use the Mailbox app. If I press and hold on some text in Text or Spreadsheet, I expect a pop up menu allowing me to clip, copy and paste. I do get a pop-up menu, but it doesn't contain those options. I have searched through the different menus, but clip, copy and paste seems to be found nowhere.

So please let me know, how do I copy paste in the Mailbox app on Android?