So, after the patch I seem to be gaining xp from respawning enemies that I'm not currently killing. I simply run past an area that I've already been and if there's a fight between two alternate factions in progress, I can just sit there and gain xp from kills. Kind of fun.. but I'm sure this isn't intended.

I can confirm this happening as well. It used to happen to me before the patch but not very often (I just chalked it up to that maybe I hit an enemy and it ran off then got killed). However after the patch it's getting much more prominent I will get several cha-chings with xp gains in a row when I know I haven't even shot my weapon recently. Oh, and it happens regardless of if I'm in an area I previously cleared or not.

The only thought I had is that it may have something to do with factions? The most recent occurrences I can think of were when Minutemen or BoS peeps were killing enemies nearby.

For me it definitely has nothing to do with factions I'm in league with. The first occurrence for me was from a super mutant, gunner battle. I really racked up some xp there. The funniest one was when I came out of a club and could see there was a swimming pool far, far below me. Thinking I would probably die, I quicksaved and jumped for fun. To my amazement I did not die.. and on top of that I started getting a bunch of xp. 27, 27, 32, 48, .. It was like I was being scored or awarded for my daring jump.