MacRumors published a story that Apple has stopped signing iOS 13.3.

Apple Stops Signing iOS 13.3 Following Release of iOS 13.3.1

Following the release of iOS 13.3.1 on January 28, Apple has stopped signing iOS 13.3, which means downgrading to that version of iOS is no longer possible. iOS 13.3 was a major update that included Communication Limits for Screen Time, support for NFC, USB, and Lightning FIDO2-compliant security keys in Safari, and more. Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date.

Apple Stops Signing iOS 13.3 Following Release of iOS 13.3.1