Ok, just for anyone else experiencing something similar, I thought I would update.
Yes, it has to be the footage.
However, I have shot footage before and after with all the same pieces (gh3, San Disk extreme sd card). All were shot at 60p in AVCHD.
The only difference that I can gather is before shooting my footage, on the same card, I shot a timelapse of the setup for the party. I’m assuming somehow that caused the footage’s timecode to be off, which in turn caused this issue.
Again, this is just speculation, but I can’t imagine what else it could possibly be.