I think I understand what was happening, and it wasn't a memory issue, which is good. When TG2 renders, it starts with the GI pass and fills in with "dots" that represent light samples in the scene. When primary rendering happens, it "erases" the visible dots for areas it's rendering. For longer renders this effect would be more noticeable and would also depend on the render tile size (which is affected by AA, in this case a fairly low value, so the tiles were large and would affect a larger area).
So in any case, as you found, it's not an error and everything works out fine. Sorry about the confusion and having to cancel a long render!