As part of storing the lastVote document, we will wait for it to be durable, and we will eventually call into refreshOplogTruncateAfterPointIfPrimary. This needs to acquire a global IX lock as part of an AutoGetCollection. This can deadlock with stepdown, as it tries to clear the oplog truncate after point, which in turn waits on a journal flush. The journal flusher needs to be able to run wait for durability too, but it cannot get to the critical section as that is protected by a mutex which is already held by the lastVote thread.

We recently made storing the lastVote document fully uninterruptible in SERVER-47612.