I just confirmed that the problem seems to be caused by the FSRS4Anki Helper add-on. Process:
- Booted Anki with add-ons enabled
- Ran through around 20-25 reviews and got no new cards
- Disabled FSRS4Anki Helper and rebooted Anki
- Studied around 10 reviews before getting a new card
Here are my FSRS4Anki Helper settings: