I was of the understanding that the long term/multiplicative fuel trim (18.8%) was a result of under load/boost?
on a wideband ecu your right.. I am not so sure though how it can be the same on narrowband, which switches off its lambda control under loads though.. block 001 will drop to zero under loads where it ignores lambda for correction. thereafter its relying on map itself and airflow, and loads to determin fueling.
the learned value on cruise is being applied across the board tho, which is resulting in over rich under boost..
when I see this during mapping, the fuel table and injector size has to be adjusted so cruise throttle is close and 001 is not trying to adjust to correct, then the underload fueling can be addressed and mapped to suit.
your ecu and map seems to have opposite ends, running lean on cruise (too large a maf if >80mm OD?) and its fuel trim is cocking up the underload fueling as a consequence.
I think craigs onto something by questioning going to smaller MAF.. (I dont remember what size you run now)