Amazon "Keep me signed in" missing?

Until a couple of days ago, when signing into Amazon using Safari on my MacBook Pro, there would be a check box labeled "Keep me signed in". It has disappeared, though it is still there on Chrome (macOS) and on iOS Safari.

I've tried purging all website data and rebooting, but out won't come back.

Why would it be missing from Safari but not Chrome, and how can I get it back?

Safari on left, Chrome on right.