Add a Wait of 0.00 second at the start of the Action.
Sometimes Events fired too fast before the Action is done, maybe that's what causing it.
NOTE: Adding a 0.00 second of Wait does not affect its MUI-ness.
Or perhaps this has something to do with stopping a Bloodlust-based ability from targeting a certain units ?
If it's a Bloodlust-based ability, you should perhaps;