...Basically, what is happening here is that for TOS, they took the airdates as is, which means a season was split between two years. For TNG, they assumed that a season mentioning 2364 in a late episode would completely
take place in 2364. Hence the erroneous application of a doctrine here.
Of course, we have many reasons to think that the TNG seasons would also be split between two Earth years: the very few Earth dates mentioned (such as that for First Contact, or the Hindu festival of lights) all systematically support a season that starts after the summer, not after Christmas.