Bypass thrust loss up high
Seems to me, I remember a GE CF34 number like 8500ish lbs thrust at sea level, 1000ish at altitude.
I did find a chart for the cruise thrust of several engines.
CFM56-7B18 SL= 18,500 CRUISE= 5905
TFE731-60 SL= 5,000 CRUISE= 1120
I do know the overall bypass ratio will come into play, as well as the dynamic efficiency of the fan itself.
Seems that the rule of thumb was that much above FL350 (area of max-Q), most of the work was being done by the core.