More info; the result of a "tmsh show /cm traffic-group traffic-group-1 auto-failback-enabled" run on each LTM.
TLDR--The active LTM does not see the standby LTM as being online. The standby LTM does view the Active LTM correctly
Active
config tmsh show /cm traffic-group traffic-group-1
CM::Traffic-Group
traffic-group-1 standbyF5.net.unc.edu offline false
traffic-group-1 ActiveF5.net.unc.edu active false
Standby
config tmsh show /cm traffic-group traffic-group-1
traffic-group-1 standbyF5.net.unc.edu standby true
traffic-group-1 ActiveF5.net.unc.edu active false
Both LTMs are synced, but on the Primary LTM the traffic group lists only the Active LTM--as "current device"
Underneath that is "Next Active DeviceInitializing / Not Synced"
Essentially, the active F5 does not see the standby as being in its traffic group, even though both appear in the HA order box, even though the standby does sees the active. The gui on each is exactly the same, save that on the standby, the correct ltm name appears as "Next Active Device"
Testing this on a virtual pair with no traffic, and I found that if I force the primary to standby, 1. The previous standby LTM will go Active
2. Then the originally active LTM goes active AGAIN
3. And the previous standby LTM falls back to standby.
So the standby LTM behaves as it should, but as far as the primary is concerned, it is the only active ltm in the traffic group. Auto-failback is NOT enabled on either LTM.
I just upgraded three pairs of virtual LTMs from 11.4.1 to 11.6.0 on our Viprions, and each pair has the same exact problem. Also, each of the Active LTMS with the issue are hosted on the same Viprion chassis, so there may be an issue on the chassis itself forcing the behaviour, but I am damned if I can figure out what it would be.