Any English experts? I would really appreciate your help.

Hey guys,

I’m a bit stuck on a part of my English essay I am writing and I would really appreciate any help.

My problem is to do with the application of the word: but.

‘I really enjoyed that lesson but we learnt a lot’ – This sentence to the best of my knowledge is incorrect as there is no contrast between the part before the ‘but’ and the part after it. Or another way of putting it, what comes after the ‘but’ doesn’t surprise you. The sentence would be correct if for example it was written ‘I really enjoyed that lesson and we learnt a lot’ or perhaps (for a different meaning) ‘I really enjoyed that lesson but we didn’t learn very much’.

My question is how come the grammatically incorrect sentence ‘I really enjoyed that lesson but we learnt a lot’ sounds correct when we write ‘I really enjoyed that lesson but we did learn a lot’ even though based on usage of ‘but’ there is still no contrast; or is there one that I can’t see?

There are other examples E.g.:

‘I got a good result but I revised for ages’ – Grammatically incorrect (I think).
‘I got a good result but I did revise for ages’ – Sounds correct but why?

Thank you for any help, I am very grateful.