Hi,
The context menu in the Music Library (and probably at other places as well) is not fully visible and thus not usable on small screens. This affects smartphone screens in landscape mode and also the kiosk mode with the Raspberry Pi display and the Touchscreen plugin. Basically, this makes the touchscreen plugin hardly usable because I cannot select the music folder to play.
See attached screenshot. Running version 2.513 on a Pi.
I don’t know about displays attached directly to the Raspi (i have connected a HiFiBerry DAC), but for iOS there is an App without limitations … for free and I don’t miss another, because there is no need to walk to the Raspi 
I have the same problem here with the web interface.
When I scroll a little bit, the menu is visible.
Android smartphone with chrome browser.
That’s iOS Chrome without Problems. It seems it is a Android Problem, but have a try to switch Chrome into Desktop Mode. May be that will change the view.
Your way is working fine for me too (search in Web Radio).
But I have added some webradios in “My Web Radios”. When I try to open the context menu of the 2nd webradio there, I see this strange behavior.
No, no problems at all, I have added a couple of Stations I’m using in my Subsonic Music Server. No Problems.
But may be it is indeed the screen size, I have iPhone XS Max 512GB, you can see the screenshot, it’s bigger than yours.
But BTW the Android App for Volumio costs 2,37€ that should be 2,50$ and is supporting the Future of Volumio. It seems that the people can buy a DAC but don’t have the 2.50 for supporting …
My problem is not Android. It is the Pi touchscreen. I just wanted to show that I see the same issue on my android phone as well.
Understand, but my answer was more into the direction to the Android user.
You didn‘t say much about your Display, there are a lot of hits in google and YouTube about Volumio and different Displays. But without additional information it is not easy …