still sounds like a filed issue.
on you monitor you would be seeing BOTH fields mixed together,
resulting in the flash seeming only half as intense.
in FCP you generally only see one field displayed in either the viewer or canvas.
but there sounds like there might be a problem.
you say you see the full flash in the VIEWER, (lets say you are seeing field "X"
but in the CANVAS you don't see it. (so you are seeing field 'Y"
this makes me think your sequence setting is not matched to your clips properties.
that might be worth looking into.
as for a solution to the imitate issue,
you CAN see both fields in viewer or canvas by setting them to display at 100%
a quick, easy way to do this is to select the canvas, and press and hold H.
this will pop the canvas up to 100%
both fields will be visible within a comb pattern.
so this way you can find the frame with the flash,
isolate that frame by blading it on either side.
now, apply a de-interlace filter to just that frame, and adjust it so you are seeing just the field with the flash.
cheers,
nick