There’s a well known old-device bug with Screen Time. Once you’ve setup up Screen Time for a child’s device it will always show up under their Apple ID — even after you wipe the device.
So my daughter’s old iPhone 6 showed up under her Screen Time Apple ID — even after it had been wiped.
Today I changed the Apple ID for that device and enrolled it in Screen Time under a new Apple ID. Then I went to look at her Screen Time device list. Lo and behold — the old device was still there, but now its device name changed to match the device name it had when I reenrolled it with the new Apple ID.
So now one device shows twice in my remote Family Screen Time, once under my daughter’s device list, once under the new Apple ID.
I’ve read that Apple’s Screen Time was a “rush job”. Looks like the dev is doing Screen Time by storing a device identifier — maybe a Serial Number and the device name used at enrollment time. They should have used a combination of Apple ID and device identifier but they used device identifier alone.