I can confirm this, seems to be a bug.I am using MP2 2.1 and the rewind seems to be broken. When I click on the rewind it just pauses for a sec then carries on playing. FF works ok.
I just tried WMP and VLC and I also can not use all FFW or REW speeds for those that do not work in MP2.1.Fast forward and rewind are "features" of the involved filters in the DirectShow graph. Especially the video decoder needs to handle this. MP2 just tells the graph to do it.
So if it doesn't work now, the only variables are the used filters. You might try others than LAV, but I guess each filter has other disadvantages, like MS ones to cause long zapping delays in TV.
I never use ffw/rew, the skip steps with left/right are much more useful
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