Odd volume drop each time I open a Pro Tools session

I have an odd issue whereby every time I open a session, where SoundID is across my master output buss, I have to go into the plugin and despite it showing the volume slider at 0.0db (as marked in the picture), SoundID is clearly dropping the volume until I manually grab the slider and move it up and down!? It sounds like it's applying the safe headroom even though that feature is always turned off! Every time I open a session in Pro Tools I have to remember to manually slide the voume up/down for it to reset. I'm on the latest version of SoundID and I've had this issue for ages, certainly on the last version too.

The odd thing is, on the first load-up of Pro Tools the output volume is correct. If I then open another session I get the issue above. If I close Pro Tools completely, re-start it, I have the correct output volume until I open another session. Even more odd, if I re-start Pro Tools, open session “My Song”, close “My Song” and then re-open it, i.e. the same session, I get the volume drop again. Therefore, I can assume it's not a session issue within Pro Tools.