I just discovered a bug in the obscure effect (modules/core/filter_obscure.c).
When it is applied on a clip that is included in a playlist, it doesn't take
into account the clip's start time, and the effect is always applied starting
from the begining of the playlist, not the beginning of the clip.
To say it in another way, the filter takes into account the frame position in
the playlist and not the frame position in the clip.
The attached patch fixes the issue (based on stuff found in filter_brightness)