I rejuvenated the 2021 HSK 1 & 2 lists yesterday.
Consequently when I went into legacy mobile this morning (as I do everyday to add new words from daily reading,) there were over 4000 words to sync. That was fine, it was accomplished in about a minute.
Once I added my new words via legacy, I switched out of legacy mobile and into the beta, in order to review and study the new words.
However, I was unable to do so as the app would not complete the sync. I waited over half an hour and my phone grew very hot, so I closed the app, reopened it, and tried again. It’s been about twenty minutes again and still no completion.
Thus, I can’t use the app.
Edit: gave up and finally deleted the app, reinstalled it. That worked. But leaving this here as I thought you should know.