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.