Without hard data I can only make a guess that during WOT its too lean and when going back to EPA mode the O2s are reporting lean so PCM is adding fuel, or maybe COT is kicking in or the tune at those RPMs when leaving PE mode has a fuel flow or VE values incorrect.
Also could be a temp issue and PCM temp tables are forcing PCM to go rich but if it clears up within 10 minutes of drive time its not a big deal.

I suspect the high IAT is driving this issue and why I suggest our water/meth injection kit since it reduces IAT about 40 degrees. That nozzle should be of course right before IAT so it can see that colder airmass

Maybe the popoff valve where it is in the path is effecting airmass or is leaking and sucking in hotter air ?
One other thing is w/meth when its comes on/off cause 02s to report rich and lean and fools PCM for a short time.
I see this all the time after w/meth goes off since when its on the PCM is told fuel trims have gone rich so PCM leans out, then w/meth being off its now too lean and PCM goes to richen up again