Thats not a bug but actually an intended mechanic, you can read a detailed explanation here for example:
https://pokemonrevolution.net/forum/topic/198189-bug-in-climates/?do=findComment&comment=1133075
Love Island is an island of love and friendship as the name indicates. You need a pokemon that loves you to break these special rocks.
At least thats the explanation I like to go with.
Its been almost a year since the Q&A-stream where Walross mentioned he will put an NPC for that feature if it will take too long to implement it into the client itself.
So it would be nice if that feature could be introduced now via an NPC since having a Membership should never be a disadventage for the player as it can be currently.