All who’ve upgraded their phone or tablet to iOS 12.3 can no longer backtrack to the previous version. This is because Apple has stopped signing iOS 12.2.
It’s not a reflection on that older version, but a standard action for the company to take. It always ends the installation of obsolescent versions shortly after a new one is introduced.
No iOS downgrades allowed
Apple has to digitally sign a copy of iOS for an iPhone or iPad to install it. It carries on doing so for older versions for a week or so after each new one is introduced in case major bugs are found in the replacement. This allows anyone who must downgrade to do so.
But iOS 12.3 has now been out since May 13, so it’s unlikely that serious problems in it will turn up now.
There’s no reason the hold on to iOS 12.2. The next version brought the redesigned Apple TV app that Cupertino previewed during its “show time” keynote in March. There were other improvements too.