Thanks for the answers to my query. I just wanted to clarify that reverb etc. variations on playback would not null. One of those moments when you think you've being doing something basically wrong in the routing and exporting of projects , like for years!

Something else to consider is what you're exporting.  When I've mistakenly set the export options to "entire mix", this sums busses together, which could result in an FX bus being louder in your resulting export - that's exactly what happened to me.  Since that, I've set the export options to "Main Output" (or something like that) - I never had the issue again.