As you're not supposed to store Lithium batteries at 100% into perpituity the Mavic 2 had built in self discharge circuitry to discharge the batteries to 60% after 5 days of no use.

Do the Mavic 3 batteries do the same thing?

yes but in 9 days.

I thought that they did, but I haven't gone 5 days without flying.

Hello there terrafrost. Good day and thank you for reaching out. The DJI Mavic 3 has a Auto-Discharge Function: To prevent swelling, the DJI Mavic 3 intelligent flight battery automatically discharges to 96% of the battery level when it is idle for three ( 3 ) days, and automatically discharges to 60% of the battery level when it is idle for nine ( 9 ) days. It is normal to feel moderate heat being emitted from the battery during the discharge process. Thank you.