Android Q's fully gestural navigation has been controversial, to say the least. Some people are happy about it, but others absolutely despise it. In its current form, gesture navigation is still very much a "beta" experience. One of the issues that arose with the new gesture system is interference with Screen Pinning. A fix may be in the works for future releases.
Screen Pinning was added in Android Lollipop as a way to "pin" a specific app to the foreground. Once an app is pinned, you can't leave the app unless you know the secret button combination. Typically, that meant holding the Back and Home buttons simultaneously. However, with the introduction of the new gestures and the removal of the navigation buttons, there was no way to exit a pinned app. So in Android Q Beta 4 Google simply removed Screen Pinning altogether, as pointed out by Android Police.
Thankfully, our Editor in Chief, Mishaal Rahman, spotted a string that explains how the yet to be implemented solution may work.
Swipe up and hold to unpin. That's the solution. This may be a little easier to accidentally trigger than long-pressing two buttons, but it should work well enough. We're glad to see Google at least has a solution in mind for this issue. The Screen Pinning situation felt like another example of how Google hadn't thought out the gesture system enough. But we have to keep in mind that this is a beta product still. Hopefully, more issues will be ironed out before release.