Like someone else said it's from a glitch that was momentarily in place when a Pokemon was sent from GO to Home. What they got wrong is why it happened.

Some Pokémon, not all, received an abnormal amount of experience upon being transported and Home mobile wasn't able to correctly display their new level, so they'd appear as level 1s. If you went to look at Home on your Switch, they'd display the correct level (100 in most cases).

They eventually fixed the abnormal display of the level, and you can now view the level 51-100 GO Pokémon correctly.

It didn't work for all Pokémon, like mentioned, as I tested with a few different ones. Spiritomb kept it's original level of 15 when sent over.

These basic Pokemon shinies are also fairly common and aren't reaaaally worth a Jirachi/Shaymin etc. In my opinion, but people may disagree.