I'm almost certain that this is a bug with Apple Card family sharing.
My iPhone (XR on iOS 14.6) and Apple Watch (series 3 on watchOS 7.4.1) have been draining battery way faster than usual the past few days. Normally they both last two days between charges, but have been lasting only 9 hours or so. I’ve tried restarting both devices, removing background processes, and updating the watch to watchOS 7.5. The same thing is happening on my wife’s iPhone (same as mine) and watch (series 6 on watchOS 7.5), even though there's not much in common with our phone/watch usage patterns.
I then noticed on the iPhone’s 10-day battery history that Wallet had used 35% of the phone’s battery life in the past week. This wasn’t showing in the 24-hour battery history for some reason, even though the battery decline was steady throughout the day. The 10-day history showed that battery usage was close to 100% on Thursday, Friday, and Sunday but not on Saturday or before Thursday.
I then realized that I had activated Apple Card family sharing a couple weeks ago and switched to it as my primary iCloud payment card, but no purchases had been made until Apple subscriptions auto-renewed this week. Those transactions happened on Thursday, Friday, and Sunday, exactly when the battery drain was happening. I use Apple Wallet NFC with other cards all the time, but haven’t used the Apple Card other than these renewals.
Based on this, it looks like there’s a bug with Apple Card family sharing that’s draining battery only on days that the card is used (or maybe only on days that it has scheduled transactions). I’ve switched all payments off of that card for now, and hopefully that will solve the problem.