IPhone 6s Plus does not offer iOS updates

I bought three used iPhone 6s Plusses for family members. With two of them I had no trouble updating to a recent iOS 14. I actually don't remember if they came with iOS 14 on them or I updated them or what, it was a few months ago. In any case, today I realized the third phone had a version of iOS 12 on it. When I went to Settings > General > Software Update it offered me to update to 12.5.4. I was curious why it wasn't offering a more recent update, but I went ahead and updated to 12.5.4 hoping that once that was installed it would offer me to go to 13, 14, or whatever. But after 12.5.4 was installed it says it's on the latest version and doesn't offer me another update. Here are the things I've done to try to get it to offer a recent iOS:

• Restarted the phone several times, including a "reset" restart by holding down the home and power buttons until it restarts and the Apple screen comes on.

• Checking my network. The phone and my computers are connected to the network and can browse the internet and download large files quickly.

• I connected my phone to my Macintosh and chose Check for Update from the Finder window. That brings up a message: "This version of the iPhone software (12.5.4) is the current version." And you can see from the screenshot below that the Finder window itself says the phone is up to date with 12.5.4.

• I used the Restore iPhone button shown on the screenshot below to reset the phone to factory settings. After I went through the start-from-scratch setup steps, it was still on iOS 12.5.4. (Perhaps reset to factory just means "use the iOS that's already on the phone but reset all of the user data and settings"?)

• I have googled a lot and don't see any other ideas.

Any idea what could be going on and how I can get to iOS 13 or 14?