Summoner continues to summon vex over and over again when out of sight and range

So playing today if I understand the functions of the Evoker correctly they are to summon 3 vex at a time. While fighting vex today in a woodland mansion with only 1 evoker on the top level of the mansion left alive the vex continues again and again to multiply. I would kill 3 only to have 6. I would kill 6 only to have 10. I may simply misunderstand the way they are summoned. But I can't believe that a player running away from the evoker. Back to the bottom level of the mansion, and even outside 50+ blocks away should continue having them follow after. And if they stay on the main level after fleeing should have to fight an ever increasing number of a mob summoned by a mob 2 floors above and totally out of sight...