optical flow is broken

optical flow applied to a HD 1080i clip with speed 25 is not smooth

steps to repro

put HD 1080i50 clip in the timeline (HD1080i)

set speed to 25

set optical flow

preview render

result, not smooth motion.

CC2019 does the same very nice and smooth.

CC2020  v 14.6 (public version) has the same problem.

system Win 10 with Nvidia GTX 970 card.