WowAce

Bartender bars on the right side of the screen cover over the quest log even though the Interface settings have both right action bars active. Settings for quest log/action bars start out correct upon login, but after playing for a while, the settings reset on their own and the quest log moves under the bars to the right and are covered by them. This can be corrected by logging out and logging back in. Not sure if this is a Bartender issue or a WoW interface issue.

This bug is caused by 2 things.

1 - Having the option disabled to use the Blizzard vehicle UI (as if you don't disable this, it uses the default Blizzard Vehicle UI and if you exit the vehicle IN COMBAT you have no bars at all until combat ends, this has burned me a few times on new alts where I'm just spamming keybinds in hopes that I live).

2 - Entering and exiting a vehicle. On exit it doesn't reset the quest log back to it's proper position.

As you mentioned, logging in / out will fix it, as well as a simple /reload ui will also fix it.  If you check the option to use the Blizzard Vehicle UI this doesn't happen.