I've created a post also in the mixxx forum but i think this is a bug and is not correct to overcome it with a workaround.
I will copy-paste the message from the forum:
I’m using a raspberry and i have different touchscreens.
With mixxx versions < 2.1.8 (QT4) i had no problem at all.
Starting with mixxx 2.2 (QT5) all my thouscreens that act like touch devices stopped working.
The touchscreens that emulate a normal mouse still works, (in this kind of touchscreens if you drag over a web page the text will be selected as if you click and drag a real mouse).
The touchscreens that act as a touch devices are the one that, if in a web page you drag over the page, the page scroll, so a different behaviour with respect to a normal click-and-drag action done with a mouse.
Switching to mixxx > 2.1.8 had broke this compatibility, now all the touchscreens that raspbian handle like “touch devices” and not like “normal mouse” doesn’t work anymore.
How can i use the latest version of mixxx on this kind of devices?
Update: same problem not just on raspbian but also on ubuntu mate