Try switching your 3-piece airlock to an S-shaped airlock. Put just a tiny bit of starsan or vodka (or even tap water for that matter) in it. When you cold crash, the pressure difference will not suck the liquid all the way through into the carboy. Just be sure to use a small amount to block any oxidation from occurring.
Yeah, that's probably why I haven't had any significant problems. The S shaped airlock mostly just lets some air bubble back into they carboy.
Makes me wonder how much air it sucks back in, though. Good thing there's a blanket of CO2 over the top of the beer, I guess.
Regardless of whether you use an s-bubbler, foil, or silicone bung when the warm air in the headspace cools and contracts you will suck in the same volume of air, making differences in "oxidation" meaningless. Except possibly the silicone bung .... you might create an air tight seal than leads to negative pressure in the headspace that then equilibrates with CO2 from the beer or allows air in when you open to transfer but with shorter contact time there might be a real (minor) difference.
When I use a blow off tube, I don't bury the opening in star san just point it downward into a bucket. Bacteria and yeast won't climb up the tube, and the generation of CO2 in the beer will keep the oxygen out. Then if I have a decrease in temp, nothing to suck back in.
The only down side I see is fruit flies. .... .... I hate fruit flies