I had a video get flagged once. Recently actually decided to do something about it. The claim is on a tiny snipped of a movie from the 90s, the claim is being made by a studio that is so far divorced from the actual copyright owners its freaking hilarious.
It's like "Paramount produced the film in Hollywood" and the claim is from "Tiny French studio." Pretty sure B in this case did not magically acquire the rights from A.
If the actual copyright holders were to dispute the video I might go "ok, fine, I'll take it down, even though I am pretty sure I can fair-use this one clip." But from some random company in another country? NO.