I had this bug in beta 1 but did a workaround to get them back. As others have said the changelog for beta 3 seems to suggest they've fixed it but looks like they haven't.
The workaround for beta 1 was to reset the data for the lock screen/always on display app. You can do this by going to Settings->Lock Screen and AOD->About Lock Screen->Tap the info icon in the top right corner (should take you to the AlwaysOnDisplay app info page) then go to storage and tap clear data. This will only reset the layout of things like the clock and such, it won't affect your fingerprints, passcode etc. This worked for me and others in beta 1 and 2 so should work for beta 3 too
Probably worth submitting a report with a log first though via the beta feedback and then reset the lock screen app data. Might be helpful for the devs