I’m fairly certain this is new, but I can’t say positively when it happened. In my schedule https://app.renshuu.org/schedu... over 200 words are can’t study because they don’t have kanji spellings. I think that they used to have them, probably rarely used kanji spellings, but not anymore. Something has changed.
I'd check your settings (in particular, the ones on the vocab section) to make sure something didn't get unchecked. (I feel like a ghost lives inside of renshuu sometimes).
It's definitely saying it is because of kanji, and not audio? Because there is/was a temporary listening vector issue today.