@Patrick_Wright You got to it before I could reply, but yes exactly that. There were many bugs and problems with layout before Bokeh 1.2, one of them being that gridplot could generate things that were not actually grids. That bug was fixed, and gridplot now only and always generates a grid layout.
Regarding:
I am not sure if SO is still being used as a primary channel for support, or if this Community Support forum is preferred?
I don’t think there is any official preference, but if I were to make a personal ask it would be that folks try not to simultaneously cross-post questions in different places (unless they are willing to cross-post any answers everywhere!)