I think the latter is a possibility too; the Japanese language share little from syntax to vocabulary to memes with other languages, including Chinese(Japanese is NOT a dialect of Chinese; arguably it was SUPPOSED TO in the ancient times, but Japan was simply unable to achieve that status), despite sharing Unicode codepoint ranges with the Chinese language due to Han Unification. So there might be some chance that Japanese part of the dataset is inconsistent and contradictory to other parts, in some ways such that, it would make sense to include just the bare minimum to achieve best use of resources available at this point. Chinese models seem to do this, and dataset fractions of Japanese content is always tiny. Some of Chinese image models even straight up un-support Japanese.

As to what teshigahara said in the sibling comment - I think that shows the readings are arbitrary rather than being simply complicated and hard to infer for beginners. It must be that the Japanese written-spoken convergence movement began in the early modern times never reached its goals, beyond the written language having its own tone like in many languages, and so the verbal Japanese expressed in Japanese writings is simply not complete.