I am lucky enough to have iTunes 12.6.5 and High Sierra installed on my 2014 MacBook Air, so I can still manage apps and custom ringtones as usual.
As for the ability to sync ringtones, there are some people saying dragging the ringtones to iTunes might just do the trick, and to delete the ones you don’t want, just swipe left on the ringtone and tap “delete”.
Since I have no plan to use Catalina (Haswell simply too slow to run it), I am fine. But I hope you guys out there can figure the issue out.
Regarding the fix, yes, in beta 2, all custom ringtones work as intended.