Yeah, Elusive monsters are supposed to ignore investigators for the purposes of movement. But when you're against Glaaki/Tulzscha, Cultists become elusive stalkers. Clearly, these two abilities can't operate fully together, so what I've been doing is that they don't ignore you for purposes of movement, but you still have to pass an evade check to enter combat. This captures the most important aspects of the two abilities.

Interestingly, I like the idea of the abilities working together..."is that someone behind me. I thought I noticed something at the edge of my lantern's light...no, it can't be, these streets are quiet at this time of night. What was that!?"