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.
That’s not a bug. The way “Effect Veiler” works is that it negates the targets effects on the field. Even if the targeted monster leaves the field as cost, the effect is still considered to have been negated on the field.