Android Q is nearly upon us, bringing with it a slew of changes and refinements. Beta 5 has started rolling out too, but it seemingly confirms that gesture navigation controls will not work with third-party launchers. The slow-march to a closed-source Android seems upon us.
https://www.notebookcheck.net/Android-Q-nerfs-gesture-navigation-if-you-use-a-third-party-launcher.427164.0.html
Removing the gesture input on third party launchers is a quick fix for stability - they're addressing the issue properly in a future update! See the actual changelog:
"Custom launchers are another area where we've heard feedback and we're continuing to work on issues, particularly with stability and Recents. Starting in Beta 6, we'll switch users to 3-button navigation when they are using a custom launcher by default. We'll address the remaining issues in a post-launch update allowing all users to switch to gestural navigation. Meanwhile, please continue to give us your feedback."