I just found this thread after I had created a similar one, so sorry for the redundancy.
I have a similar issue, except the expression stops working completely when disabling multiprocessing.
If I scrub on the timeline it doesn’t work.
If I ram preview with no cached frames it works.
If I render immediately after it showed up on the ram preview it doesn’t work.
If I Command-ram preview the saved movie doesn’t show the expression applied.
If I bake the expression it doesn’t work either.
Did you ever solve this issue?
audio=thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider")
if (audio>=.5)
{wiggle(5,20)}
else
{0}
;