A discussion with several fans on the Rhemuth Castle website describes several theories of the development of Deryni hatred in countries neighboring Gwynedd. Perhaps, "After the Deryni backlash in Gwynedd, many of the surrounding Kingdoms and states downplayed their Deryniness." Or, perhaps, "after Festil I seized Gwynedd and enacted some rather oppressive laws to protect his Deryni supporters, that there was a wave of departing objectors, at least among those with the resources to leave - such as nobles and clergymen" who were absorbed into, and influenced, the local church.
The only information we could find in the books themselves is in the Codex Derynianus:
The Princess Jehane was educated at the Convent of Saint-Elie new
Millefleurs; the Abbess of this establishment, one Mother Rohane, was
a notorious Deryni-hater.
I did not find any entry in the Codex for Mother Rohane, so there does not seem to be any explanation in the existing books, about where this Abbess acquired her Deryni hatred.
One more hint is a comment, in Camber/Alister's thoughts shortly before Cinhil's death, implying that Gwynedd was not the first to experience this process (emphasis mine):
Now the anti-Deryni factions were about to get their wish. Cinhil
would die within the year, probably within the month, if Rhys’s
estimates were correct, and young King Alroy would be ruled by his
regents. The last of the Deryni loyal to the Crown would be ousted
from their offices, their positions of influence, no matter that many
of them had served Gwynedd and its present king well and with
distinction. And then the ostracism would begin, and the persecutions,
and finally the bloodshed. It had happened before, in other lands, in
other times.