That's no surprise, with the 2n5771's relatively high capacitances,
3pF, compared a much lower 0.3 and 0.55pF at 5-10V reverse bias.
About 0.7pF at sensible reverse bias according to the datasheets,
just a bit higher than you've been using. But there are plenty of
lower-capacitance smaller-die-size choices if that's important.
There may be plenty (readily available? in a through-hole package?),
but judging from the replies so far there isn't plenty of knowledge
about them and their uses ... Dunno if that makes it important ...