Master Isolator volume adjustment isn't reflected in the [SEND FX] effect sound. Why not?

This is part of the specifications of the product.

When an effect is applied using the BOOST SEND function, the effect sound is added at a later stage than the Master Isolator so that the effect remains, even when the Master Isolator is cut.

As an example, if an effect is added to the audio of Channel 1 by operating the [SEND] knob and the [LOW] is set to minimum in the Master Isolator, the [LOW] of the sound from the channel after passing through the fader will decrease in volume, but the [LOW] of the effect sound will not change.

For details, refer to the [Block diagram] under the [Specifications] in the Instruction Manual.

You can read the Instruction Manual [here].