Chutney networks need at least 3 AssumeReachable nodes.
Unless at least three nodes on a chutney network are AssumeReachable, the consensus as published will not have 3 descriptors in it. Without 3 descriptors in the initial consensus, relays won't be able to build self-testing circuits, decide they are reachable, and publish their own descriptors... so they will never join the consensus.
Right now, this problem is masked by #34446 (moved), which means that we haven't actually disabled AssumeReachable as much as we think we have.