That top one isn't an abrupt phase change. Instead, you are seeing the phase transition through 180 degrees, so the display snaps from -180 to +180, which is actually the same value. It's a display thing, not a phase behavior thing. If you were to offset the phase by about 90 degrees the two displays would look fairly identical. [edit: no they wouldn't]
It's still possible that associated impedance change of the diplexer is causing your amplifier oscillation, but again, this is not an abrupt phase change.