In a similar fashion to this bug report, I have discovered two more methods that messes up /back (that previously didn't mess it up)

1.) Hitting the world border messes up /back (tested it in the nether, but I assume it is the same for all worlds)
2.) Using /j, which was always separate from the recordable teleports for back, now treats every instance of /j as a teleport that is logged in /back

 

Both should be good @ClarinetPhoenix , but I discovered another potential one:

When you go through a town-to-town portal, it treats the location right before entering the portal as your /back. I don't remember that doing that, but I could be wrong (could possibly be the same for the portals to other worlds like the mining or nether portals)

 

@ClarinetPhoenix I found another issue that is messing up /back.

It is similar to the world border issue; It is creating a new teleport whenever I hit an "entry deny" town's border (for example, the town of Victoria on Main) and removing whatever I had on my /back