If I understand you correctly, you have built a base not far from where a large white dragon walks, blocking the only road to the south? If so, that might also have an effect. The program does not like NPC spawn near world bosses. My last base was in an open area not far from the world boss, and the attacks came from everywhere except the direction where he was sitting.
And another reason for the purge spawn inside the base may be its too large area or length.