On an almost-daily basis, my iOS devices stop synching Habits with the web site. This happens with both my 7th Gen iPad running iOS 15.4.1 and my iPhone 13 running the same iOS version. However, this has been a problem for years, running across several hardware platforms and multiple iOS versions.

To restore synchronization, I have to restart the device. It then works fine, for a few hours to (occasionally) a few days, then it has to be restarted again.