This report comes mostly from code inspection. When chaining is not allowed in replication, ReplSetImpl::getMemberToSyncTo only allows a secondary to sync from the primary. If a primary cannot be reached, syncing does not happen.

In consensus.cpp, an election will refuse to elect a member with a lower priority if a member with a higher priority exists and is within 10 seconds of being caught up.

These two facts together can cause a replica set to never elect a primary.

Take the following scenario. Chaining is disabled, and no primary exists. Member A has priority 10 (the highest in the set), and is 5 seconds behind member B that has priority 1. B is furthest along. Neither A nor B will ever be elected. B won't be elected because the election algorithm will say "A is within 10 seconds and has a higher priority". A won't get elected because it is behind B, and because chaining is disallowed, cannot replicate from B to catch up.

I think the end result is a primary never gets elected.

I don't see any code that says "ignore the chainingAllowed bit and replicate off a secondary because a primary does not exist".