After the release of iOS 15.4, Apple repeatedly signed iOS 15.3.1, which came out unexpectedly. iOS 15.3.1 is no longer found, it is impossible to roll back to this version of iOS if you have updated to iOS 15.4.
If you have upgraded to iOS 15.4, then you will no longer be able to downgrade to iOS 15.3.1 as it will no longer work, says IT-Here.
Apple usually stops signing old versions of updated software after new ones are released to encourage customers to keep their operating systems up to date, which is why iOS 15.3.1 is no longer picking up.
NIX Solutions notes that iOS 15.3.1 was updated with a bug fix that fixed the WebKit vulnerability, while iOS 15.4 was implemented with an update that brought new Emoji, Face ID with disguise detection, and a host of other features. Apple has also stopped signing tvOS 15.3.