Unable to pair Apple Watch SE 1st generation with iPhone 8 Plus

I'm getting the message that iPhone XS or newer is required, even though the specs say the Watch SE gen1 is compatible with iPhone 6s or newer (and someone in the family has the same combination working fine: Watch SE gen1 / WatchOS 9 + iPhone 8 Plus / iOS 16).

I suspect the Watch SE gen1 may have been upgraded to watchOS 10, and once upgraded there is no easy way to downgrade - or is there? Actually, the other Watch SE gen1 was prompted to be upgraded too, but the upgrade was refused with "please upgrade the iPhone first" (impossible because it's already the latest iOS 16 and newer iOS is not available for iPhone 8 Plus). Kind of logic error on the part of Apple, as the Watch app should know what device/OS it's running on and not ask the user to do the impossible thing (upgrade to newer OS that doesn't exist for this device).

I'm still happy with my iPhone 8 Plus and don't see a good newer replacement available (either no Touch ID or smaller size in SE 2020/2022). Is there a way for Apple to downgrade the Watch to the original watchOS 9? I can still return it to the seller if there is no other way, it was sold as "new, post-exhibition" and came in the original packaging).

[Re-Titled by Moderator]

Original Title: Can't pair Watch SE gen1 with iPhone 8 Plus