Effect Veiler continues to negate monster effects, even when they tribute themselves as cost.
Bug reproduction steps
Summon “Myutant M-05”, opponent negates with Effect Veiler. Activate "Myutant M-05"s effect to tribute itself and banish a spell from hand.
"Myutant M-05"s effect should not be negated and “Myutant Mist” should be Special Summoned.