Wifi networks removed from one device still appear on other devices

Using all iCloud features to sync my data. Over the years I have accumulated a large list of wifi networks, which I now want to trim down to just my home. So my iPhone now has three entries. That removal *should* sync to my other iCloud-connected devices. However, on looking at all the other devices they still have retained the long list of networks.

Logging out of iCloud and back in again seems to make no difference. Restarting the device makes no difference.

I see this as a security concern.