At the very least, the rule has been relativized through recent events thanks to Mr. Baird and Mr. Abrams.
But even before that, I'm inclined to say that an odd-numbered movie like TSFS was much closer in terms of quality to the group of "good", even-numbered movies than it was to TFF.
Let's say that an even-numbered Trek movie just had a higher likelihood of being "good", but it was far from being a certainty. And good/bad shouldn't be used as absolutes anyway. Films like TWOK, TVH, or FC were good movies, TSFS and TUC were average movies, and TFF, INS, and NEM are examples for bad movies.