Oh, the whole "Twice nothing is still nothing" was the very first thing that crossed my mind when I read that
It does feel as if something has changed with regards to these drops, though I can't exactly say what. Just feels different. In the week or so before the 3.0 drop, I was hitting these pretty hard so as to make use of the mat traders, and the reliable methods were quite reliable.
Since 3.0, it seems to be very much feast or famine in just getting them to drop, period, to say nothing about what they actually contain. As you noted, I think I too have only found one in a shipping lane. Anecdotally, it seems as if degraded signal sources are far more common now than they ever were before, and while there may possibly be more USS spawns, the percentage of HGE I have found seems to be down over all.
I concur, it's a gut feeling, but it's a strong one.