[Glasken]

Not up to speed with melee tactics, but a clarifying question:

When you start BW, and the target moves, does BW instantly fail, or does the BW animation/timer continue and only show the failed message at the end of the attempt?

Example:

Begin BW
Target moves
BW continues
BW continues
BW finishes
Your target has moved!! BW Failed.

or

Begin BW
Target moves
Your target has moved!! BW Failed.

If it is the former, and logs show BW failing immediately after taking damage, that is a strong case for damage being what interrupts BW, rather than just movement from being struck.