I just created my own thread (oops), but searching now brought me here. I just encountered this bug too. Version 1.0.0.194 (313929), with the Aztec DLC added.
I'm 347 turns into my current game. I have been on-and-off allies with Germany throughout the game. At one point, while we were allies, some strange things happened with City States that I can't fully remember. Either Germany declared war on a City State that I was Suzerain of (either Jerusalem, Seoul, or La Venta) , or two of our city states fought each other for some reason, I can't remember. But in the notifications on the right side of the screen, I see "Germany has Declared War on Victoria" (me). I was a little confused, so I checked the diplomacy page for Germany, and we sure enough we were still allies. I thought nothing of it and assumed it meant to say "Germany Declared War on (City State I am Suzerain of)" and that it was just a little inconsequential bug.
Now, many turns later, a German unit passes through my territory, and the icon comes up for my city to range strike it. I try it, and it works, and damages their unit. I test more, moving one of my units nearby, and they are able to fight each other. Now, I can see their units massed outside of my borders. This is pretty gamebreaking for me, because I needed to rely on not needed to defend from that civ, since we were allies.
Updates from Later: I reloaded back to when it appeared, and left the German unit alone. It wandered around and attacked whatever unit it could find, but didn't attack any cities.
Also I can now confirm that at the same time this German unit was wandering through my territory and attacking things, Germany was conquering a City State I was currently Suzerain of.
I am also the type of player to frequently check different diplomacy screens and menus while I am waiting for the AI to finish their actions, and it could have happened from that.