I mean...it could be, but that doesn't necessarily make it your problem - it should be able to handle loading errors. Unfortunately, there is not a shred of error data on my end...
I do have a phone that is much older than the one you are using - let me try that and see if it works.
Additionally, I do hope to test out that system I described to you earlier, but any non-app (crossword) work is basically sidelined until back half of October, after the move is finished. Time just isn't what is was a month ago.
You don't need to bother about this right away, this is just an update from my end. I know your schedule is tight right now.
So far Hiragino would work for a 2-3 days until it failed to load again. In that case the logout/login method worked sometimes and if it failed, restarting my phone would do the trick in getting Hiragino back to work. The font stopped working again yesterday (or the day before) and I haven't attempted to bring it back. But... yesterday my phone ran out of battery and turning it on today left me with some default font still.