That's an interesting observation, growlers you don't experience oxidation but bottles you do? I wonder what the difference is between the two? I bottle off the tap by just turning down the flow to trickle in and cap on foam but typically it's one or two to take over to friends to be consumed same day. I did bottle off a keg of Warlock a few years back my buddy and I split and used a bottling gun, but didn't have much success with it. I think that was more a factor of the keg and bottles not being chilled.
I bottled off an imperial stout and Westy 12 clone about 3 years ago directly from the tap. I put a case of each in the corner of my basement for a year and put the rest in the fridge around Christmas. There was definitely oxidation when I opened up the boxed cases a year later.