TL;DR: It’s most likely a bug.
Pokemon GO simply added Turtwig and several other different Pokemon to the listing of accessible shadow Pokemon. People have beforehand decided via information mining that when a shadow Pokmeon is added, the inner title for the traditional Pokemon modifications. In this case, the inner title
TURTWIG would get replaced with
TURTWIG_PURIFIED are added. It is probably going that the identical was completed right here and that a coding mistake made the brand new regular Turtwig appear to be a distinct Pokemon from the unique Turtwig.
People have reported experiencing the identical factor in this Reddit threadand have stated that typically once they attempt to catch that Turtwig the sport crashes, and in the event that they do catch it the Grotle picture within the evolve button can be a silhouette even when they’ve beforehand developed one other Turtwig.