monitor fails on adfs proxy

i have a customer that is currently upgrading their adfs proxies. Their plan is for the servers to not be a part of the domain, but the BigIP then fails to reach them(health monitor fails, and ping also fails). If the upgraded server is rejoined to the domain, the health monitor works just fine.

Is this by design, or are there ways to make the communication work, even if the servers aren't part of the domain?