BM,

I agree.
Have a long lasting model career and also never experienced this, at least not to the point I noticed.
Flew several bipes, models and 1/1 ones the like.
Along with a professional career as an airline pilot, plus thousands of hours flying GA airplanes I do understand the groundspeed/airspeed effects, which is not the case here.
(btw I drive a motorcycle as well )

I thought the behaviour was interesting enough to spend a topic on the matter....

Ground effect is indeed a mysterious thing.
Years ago I had another strange phenomena with a Graupner Delta, I think it was an X-2000 or something.
Installed a hefty prop engine at the back, plus retracts. Ended up so heavy it failed to t/o when the grass was just a bit too long, or on a windless day.

One time I got so frustrated and let it go till almost the last part of our flying field, riding nose high and cutting grass, not wanting to lift off or accelerating more! At the very last moment I cut the throttle but to my surpride it leapt into the air as a result!

Figuring it out, the speed plus high nose angle did create enough ground effect to get airborne, but the prop was sucking the air pressure away underneath the wing. The moment I took the power away, the pressure recovered and pushed the model in the air! Almost comical to witness. Needless to say I was so surpised I forgot to add power again and it ended up in the next field.
Lost the model soon after that, If I remember correctly due to some interference.