For the past couple years now, across multiple phones (S9, Pixel, OnePlus 8T), when I receive a call that I don't want to answer at the moment, and hit the red 'decline' button, the call prompt disappears for half a second, and then the phone keeps ringing. Obviously a serious bug somewhere along the line, as there isn't a legitimate reason for the behavior. As it's across multiple different phones, I must assume that it is on the network side.
