Early on in my career I was told the very same thing ref thrust lever closing at altitude. The Instructor pilot explained that with the decrease in RPM also comes a change in exit guide vane angle so as to keep undisturbed flow thru the core. These were PW4056's and I think the FEGV's were scheduled with fuel muscle (happy to stand corrected) and were more suseptable to surging at alt than the RB211's that we also operated.
Perhaps this is where your answer lies.
Rgds