Filed this a while back since sprites were released, got a confirmation in the bug report ticket that this is repeatable, but strangely enough no resolution occurred.

Using Maskeraith Cloak makes all monsters ignore you except oilers, who still follow you around. They won't attack until you're visible.

(I also believe Lumbers have an unusually long line of sight and will walk pretty far to track you down even if you are several screens away, but haven't tested this thoroughly.)