Apple Watch Series 4 + iPhone XS Not Staying Connected

I've done some research to determine what indicators on the status screen indicate varying levels of connectivity between the Apple Watch and the iPhone. So I now understand that without the green "phone" icon on your status screen on the watch, your watch isn't connected (via Bluetooth) to your phone.

What I have is:

• iPhone XS Max -- two of them -- different iCloud accounts
• Apple Watch Series 4 -- two of them -- each one is paired to one of the iPhones

I got my new devices, restored them from iCloud backups (upgrading from iPhone 8 Pluses / Series 3 watches), and paired the watches to the phones. The initial pair resulted in a successful Bluetooth connection between the watches and the phones.

Then, after some period of time, even without taking the phone out of range of the watch, the watch disconnected from bluetooth. The watches now fall back to WiFi. They are all on the same WiFi network as the phones, so they get a partial connection, but certain features like health data can't be transferred over WiFi, and also, far as I know, WiFi uses more battery on the watch than Bluetooth.

Rebooting the phone/watch does not solve the problem. Toggling Bluetooth and Airplane Mode on both does not solve the problem. The watch can update network-connected features on widgets/complications using the WiFi connection, but things like watch face settings edited in the Watch App on the iPhone never get synced to the watch because it lacks a direct Bluetooth connection to an iPhone.

It seems like there is a regression in connection reliability that is causing Watches and iPhones to stay disconnected from one another, persistently, after the first connection drops. This never occurred with my last-gen devices on iOS 11, but I am not sure if the problem is iOS 12 related or device related.

But in either case, the problem is affecting two separate pairs of Watch/iPhone so it isn't just one device. Both watches and phones are not the exact same model: one's an iPhone XS Max 512GB black, the other is an iPhone XS Max 256GB silver. One watch is 44mm cellular and the other is 40mm wifi.

How can I fix this problem?

For some reason, with the latest Apple devices, if you immediately pair your new Watch to your new phone on initial setup, the Bluetooth connection doesn't appear to "stick" beyond the initial connection. The first time your phone goes out of range of your watch, will permanently break the connection.

Once you unpair and re-pair your watch to your phone, the connection will start working properly again. So you must pair your watch to your phone after your phone is completely set up, in order to get the connection to stay.

Also, it will give you an Unpairing Error when you attempt to unpair from the phone side. You then have to completely factory reset your watch, then restore the watch from an iCloud backup when you re-pair it. This is the only way I've found to fix this problem, and so far the fix appears to be permanent.