Vertical layers not adhering

I mocked up a simple print for a rectangular mold, nothing fancy. I realized very quickly that the walls are far too thin in this design and I'll have to re-do it thicker; no problem.

I was a little surprised to see this happening, though:

Looks like the walls were two "units" thick, and they split right down the middle, not adhering to each other at all. Clearly the design flaw here is the thin walls, which I will fix and redo, but I wonder whether this kind of behavior is indicative of any other problem that might need addressing? Using pretty much a stock SD2 pro on default settings, 105° bed, 215° extruder, fresh tape.