Advanced speakers understand that one meaning of ‘a pair of’ is ‘two’. So here both ‘are’ and ‘is’ are fine.

You can get away with saying "a pair are", as Ahmadi explains, but I still advise you not to do it. If you want your sentences to sound grammatical, say "a pair is". If you want to say your earbuds ARE perfect, then say "THEY are perfect". When learning a language it is best to follow the rules. Many rules are broken in practice, but make the rules a habit first, and break them later, if you still want to.

I believe it's wrong. However, it is very common in real-world English, particularly in speaking, to be influenced by the word that is closest to the verb, instead of the word that is grammatically attached to the verb. By the time you say "earbuds like these," some grammar engine in your brain has forgotten the phrase "a pair of." It is correct and natural to say "earbuds like these are perfect," so you just sort of automatically say it. One of the characteristics of a proficient English speaker is the ability to hold long structures in their head so that they are still aware of the beginning when they get to the end! Again, I think it's an error, and I notice it because you've written it down. In everyday conversation, I don't think I would even notice it.

The sentence "A pair of earbuds like these are perfect" may seem a bit unusual because it appears to have a subject ("A pair of earbuds") that is singular but is followed by a verb ("are") that is plural. However, this sentence is actually correct, and it involves a subject-verb agreement concept known as notional agreement. In English, sometimes the choice of verb (singular or plural) can be influenced not only by the grammatical number of the subject but also by the meaning or notion conveyed by the subject. In this case, "a pair of earbuds" is treated as a single entity, even though the word "pair" is technically singular. When we talk about "a pair of earbuds," we are referring to one set or one unit of earbuds. The word "like these" further emphasizes that we are talking about multiple earbuds within that single pair. So, the notional agreement here is that the subject is considered plural in meaning, even though it may be grammatically singular. Therefore, it is correct to use the plural verb "are" in this context. Here's another way to think about it: Imagine you have a pair of shoes. You might say, "These shoes are comfortable," even though "pair" is singular. Similarly, you can say, "A pair of earbuds like these are perfect," emphasizing that the earbuds within the pair are what you're describing as perfect.

I think a lot of people use PAIR as singular or plural, without knowing which one is technically correct. So, either the grammar checker is taking that into account, or it doesn't know the difference either.