audio mix duration bug?

I have a soundcue that is playing when a grenade explodes.
I make a soundclass called “explosions”, and attach the cue to it.

I have background sound in a level that i want to duck when the grenade explodes.

I set the background sound to a soundclass called “BGSound”

I then create a soundmix called “DuckOnExplosion”.
I double click that soundmix and add the member called “BGSoundmix” and set the volume adjuster to 0.5
under soundmix i set the duration to “1”.
The problem is that no matter what number i type in, under duration, the ducked soundmix always lasts until the explosion sound is finished. With the tail of the sound, that is about 3 seconds. Not 1 as i typed.
It is like it is stuck in a position called “apply until sound has finished playing”, and not what i defined in the “duration” time.

I have attached a screenshot.

Is this some setting here on my end, or is it a bug? It has been like this for the past year of unreal versions.