Just to add I get this too on my iPad Mini 5. @noris - as you say it is at random; oftentimes when I forget to go landscape before opening Roon it does open properly regardless but every so often it does this. Doesn’t bother me too much - just feeding back the seemingly random nature.
Next time it does it I will try and remember to look at how many apps are open in the background and which ones and see if I can consistently replicate the behaviour.
I am pretty sure it would happen with my 10.5 Pro also but I rarely use that for Roon 