I don't really agree that adding "did" to these sentences makes them any more grammatical.
Talking about a sentence as being grammatical can be tricky, though, because you can often reinterpret the sentence in a different way or think of a situation in which it might be grammatical.
For example, ‘I really enjoyed that lesson but we learnt a lot’ would be OK if the speaker has the belief that it's not possible to enjoy a lesson and learn from it at the same time. Certainly there could be people who think this way.
I can't see how adding "did" would change that, though.
Student or Learner