Issue with json_encode() while using 7.3

I use json_encode() on my site. My site crashed because json_encode() somehow became undefined while using PHP 7.3. I downgraded to PHP 7.2 and it fixed the issue.

Any theories on why this would happen? Could it be that an auto upgrade to a 7.3 minor version changed the json extension to one I didn’t have?

How to protect against this?