I have heard ppl say because the Ba'al Hamachsir of Bartenura is a Meshichist.
FYI I drink Bartenura (or I hold of drinking it )
I happen to have heard, first hand, at least one Major litvishe posek in NY say this.
He happens to have mispalelim/talmidim who come from, and still have family in Italy. He did his investigations, and that was his conclusion.
I heard this at least 10 years ago. I do not know what he says today. They have since added additional hechsherim.
As I recall, his reasoning was not that a meshichist will cause a problem with the wine, rather that there are problems with ne'emanus, and eidus.