First, I'm not sure how much learning precisely you were expecting from the troll in this limited period of time, most of which was taken up by it feeling fly-bites and smacking around the flies, nor even how you would expect for such to be seen.

Secondly, it did seem to learn. George hit it with three Ventus spells, each one moving it further towards the edge of the terrace. Between the second and the third, the troll dug its hand into the stone, anchoring it in place so that it would not be blown over the edge. If that's not adapting to match a new threat, I'm not sure what would be--certainly not in the brief time of the fight where most of the attacks were on the level of fly-bites.

If that is not displaying a learning ability, I would like to hear an example of a learning ability that it could have displayed.

I... think that the effects there would actually be much worse: The troll would be basically stateless. It's not even clear how that sort of thing would avoid disrupting the transfig. process.

Perhaps it's somewhat more advanced, like the charms that McGonagall was mentioning.

This is if the spell made logical sense when carried out to the fullest. But, magic doesn't work like that, it works the way we would naively think if we said "transforming back into itself."

It does sound like exactly the kind of clever hack Harry would use to get an indefinite healthy lifespan, though.

Hermione would probably maintain it. Or maybe someone else. Harry should probably be doing this to himself, too.