Recently, I came across a comment where a native English speaker stated that it is not "you are welcome" but "you're welcome". It was a side-comment as the original post wasn't about their difference; thus, there wasn't any further explanation for that.
I have always thought there isn't any slight difference between the two versions such as in other sentences with contractions with the verb be except that they would show more emphasis on it when it is in its full form.
After searching about this, I've ended up with one possible reason why "'re" may be more preferred than "are" in that specific phrase. It is because it's more usual to be used among natives. Ngram supports that too
So, is it a matter of being usual and unusual? Or does it have other issues that make it not a phrase to use when expressing thankfulness?