Just did a little testing with this. It seems not only to be related to the export function.
Basically i created 2 group tracks, on one i loaded Decapitator and on the second i loaded Fabfilter Saturn and then on the master bus i loaded Fabfilter Pro-L2.
Now if both plugins on the group tracks are disabled, Pro-L2 shows -INF at the peak level meter but at soon as I enable one of the plugin it will show -167Db in the peak meter of Pro-L2.