iRule LB_FAILED

it might not be the answer you are hoping for, but have you tried?

if you can't because of chance of disruption then try on a test virtual server. but even without attaching creating the iRule will at least do some syntax checking and determine if certain commands are allowed in certain events.

if you can't try have you searched the iRule clouddocs page?

https://clouddocs.f5.com/api/irules/LB_FAILED.html

throws up some interesting info: Note: The LB_FAILED event is not triggered for Performance L4 virtual servers. There is an enhancement request to add this in BZ341776. Contact F5 Support for additional information.

https://clouddocs.f5.com/api/irules/LB__reselect.html

seems to indicate that just LB::reselect selects the next pool member, which doesnt exist, so at least it has be be

LB::reselect virtual VS_Any_Bypass