Expected result is that the items stop at the edge of the disabled belt and don't get pushed in random amounts during that entry tick. I'm looking for a same kind of behaviour that you get from placing a missing belt as shown below. The sides of the belt line up perfectly but unfortunately this behavior can not be automated.
The lamps on the setup are configured to flash when they get a pulse of exactly two wooden boxes. You can notice the inconsistent behaviour on the top belt. The light doesn't flash since it receives two pulses value of 1 instead of a single pulse.

In order to reproduce this issue the items should be placed on both sides of the input belt at different positions since items put at the same position will get pushed into the disabled belt the same amount. This is a race condition and may require few trials.