Glad you found it. It's been a feature for over a year or so and maybe longer now. It was driving me crazy before the patch, for sure.
I am willing to bet that your object is confusing the slicer fairly bad, or at least, making the original issue you had look much worse. Any path optimization got destroyed by those comb-like features, I bet.
The downside to this (especially if your PID tuning is off) is that continuous printing of a large top layer can suck a ton of heat from the nozzle. In the worst cases, you get a jam and risk grinding your extruder as the filament stops melting fast enough. The "normal" mode of offset lines gives the hotend a bit more time to recover between sections. (I believe Revo-style hotends mitigate most of that problem because of the uniformity of heating.) Still, this problem is fairly rare but it has happened to me on occasion.