What is it called when two unrelated clauses are joined and make no sense

I’m find this very hard to describe, but as it’s something that I find grating I’m trying to find a word to describe it so I can discuss it with people.

Here is a conversation that to me makes sense:

Me: Do you like butter?

Someone else: No I don’t, but I can spread it

This is not jarring. Maybe they could have said ‘but at least I can spread it’, but that feels implied.

Here is an example where the two clauses feel jarring:

Me: Do you like butter?

Someone else: Yes I do, but I can spread it

Here, you expect but to be a counter to the first clause, but in this example it’s not countering anything. It feel clear the speaker meant to say ‘and’, but regardless this formation feels jarring.

Here’s another example:

This is not isolated to but, however I can’t find good example.

What is this called?