I had this same issue in 11.2.0, which persisted after an upgrade to 11.4.0HF3.
In my case we didn't have a dedicated heartbeat between units, which we added on the premise that going through our switching gear may have been mucking things up(and it was a good idea in general). It didn't help. Rebuilding the trust relationship also didn't help.
The resolution for us came after one of our C2100 blades packed it in and needed to be replaced. After configuring management the replacement blade and rebuilding the trust between the VIPRIONs, the issue mysteriously disappeared.
I suspect the hardware, since the C2100 blade on the other chassis also stopped POST'ing about 4 months earlier and replacing it in the same manner didn't resolve it. F5 couldn't find anything amiss with our configuration.