Soundtrack only plays with narration it's not supposed to!

I don't know what's going on I've not had this issue before, I'm wondering if I've done something wrong somewhere somehow...

I have an .MP3 soundtrack I use as background music, I've used this lots of times in Premier Pro 2024 no problem. I then do narration over the video clips with my USB microphone using the Voice-Over Record option in Premier Pro 2024, and I set the Audio Gain on the music to -23Db and then adjust it lower a bit just so it's not too loud depending on speech volume.

I've done this for many video's in the past and it all works. This time after processing, it's decided to have the soundtrack volume link up with the narration somehow, so when I talk the music is playing, when I stop talking, the soundtrack volume dips and in most places it mutes it completely, until I talk again the volume of the soundtrack comes back up. 
This even happens when I stop talking for 1 second to start a new sentence (still on same narration audio track) the music will dip down for one second and resume when I'm talking!

I've NEVER had that happen and I cannot for the life of me understand what's going on, I've deleted the .MP3 and added it in again, makes no difference! It's driving me crazy I can only assume I pressed a command key on the keyboard by mistake or something that's made it sync the music with my narration, I don't want it to, I want the music to play at the same througout I don't know why it's doing this

Any help most appreciated!!

James