Not really a surprise, just the transient vs the steady-state case. Flaps mostly (nearly always) produce a steady-state nose-down trim change, but in some aircraft there is a transient nose-up trim change due to a combination of a variety of factors ranging from the changing downwash over the tailplane to the different pitching moment of the flapped airfoil. The transient effects are (as always) less noticeable on larger aircraft because the greater moments of intertia damp the transient before it can do much.
I was always taught to hold the pitch attitude with stick pressure and then retrim once the transient had subsided - I always assumed this was universal.
PDR