Focus Status Syncing Across Devices with Unique Apple IDs

My husband and I use Family Sharing and have unique Apple IDs. When Share Across Devices is toggled on, his phone syncs my Focus settings. His phone doesn't seem to indicate to him that he's in any kind of DND mode (i.e. no icon at the top of the screen), so he has no idea when this is happening until he misses alerts. However, I can see that he's in DND mode because Messages shows that notifications are silenced, and he does not receive alerts.

We're not sure when this started, but he's complained about missing calls/messages on and off for some time.

Before Family Sharing existed, we shared a single Apple ID (mine), but created a unique ID for him several years ago, which he's been using since. He's even changed devices since then. This doesn't affect other family member's devices on Family Sharing.

Is it possible that there's some remnant of my Apple ID on his phone that's causing this Focus sharing?

How can we correct this?