I don't know if this can be considered script bug or npc, but there is a way to have a Pokemon from another region on your team, basically on events like Summer, I discovered that if you go to the event island (i discovered this on Summer, Vulcan Island) if you leave your team with 5 Pokemons and catch a Pokemon from there, it gets added to your team, and when you let yourself lose and get teleported to the Pokemon Center, you still have the Pokemon on the team (To clarify, i didn't entered Vulcan Island PC, so i was sent to Olivine's PC, and the Pokemon i caught was a Ralts from Kanto) so on this way new players could get mons from another region on their current region, bypassing region lock. 

Hello @Gumbinoash. My apologies for the late response.

 

What you are describing is intended. Since event islands are counted as Kanto, any Pokémon caught on them are also considered Kanto. So, if you return to another region, in your case Johto, in which you have not yet received the 8th gym badge, it will be region locked since the player does not meet the requirements to use Pokémon from the previous region yet.

However, in the rare cases this does happen, Staff are able to assist players by removing these region-locked Pokémon from their party and into their PC. For those requests, please use the Pokémon Stuck in Party megathread.

Hope that clarifies things. If you have further questions, please let me know.