Found another issue: If I have an existing all-day event in Google calendar that's marked as busy, it is ignored by Cal when it checks availability. It still shows that time as available. However, if I create a new all-day event and mark it busy on the same Google calendar, then Cal finds it and properly marks that time unavailable.

Thank you for flagging Ed! I too am experiencing this issue and shared with the team internally. We will look into this and someone may follow-up to ask you more questions about what you are experiencing!