It looks like Sound Mix EQ is always applied to all sound classes, despite any settings.
Here are repro steps for a potential bug:
1 - Create two Sound Classes, e.g. Music and SFX, each with Apply Effects enabled
2 - Create a Sound Mix
3 - Open the Sound Mix, enable EQ, and assign SFX Sound Class to this Sound Mix in the Sound Classes section
4 - In level BP add Push Sound Mix Modifier function, assign the Sound Mix created in point 2 to it, and connect it to Begin Play
5 - Add two sounds to project (Sound A and B)
6 - Assign Music Sound Class to sound A and SFX Sound Class to sound B
7 - Add both sounds to the level, for example as Ambient Sounds
8 - Play
Current Result: EQ is applied to both sounds.
Expected Result: EQ should be applied only to sound B.
Basically the same thing happens when I try to use the Passive Sound Mix.