Found during ITBLL testing. AM in master gets into a state where manual attempts from the shell to run the balancer always return false and this is printed in the master log:
2018-10-03 19:17:14,892 DEBUG [RpcServer.default.FPBQ.Fifo.handler=21,queue=0,port=8100] master.HMaster: Not running balancer because processing dead regionserver(s):
Note the empty list.
This errant state did not recover without intervention by way of master restart, but the test environment was chaotic so needs investigation.