Hello,
Myself and a few other users in this thread:
http://forum.cockos.com/showthread.php?t=182969 have found that soft takeover does not work when assigning hardware controllers to functions within Reaper.
In short, soft takeover does not work and parameters jump to where the knob was placed last.
I have tested in the last version of 4, the first of 5, the latest of 5, and others in 5.25. All have the same issue.