Just as an FYI: after 12 days, my problem has returned....
and rebooting alone does not solve it.
Summary:
Ping success from Client using 5gz-1 to Client using 5gz-1
Ping failure from Client using 5gz-1 to Client using 5gz-2
Ping success from Client using 5gz-1 to Client using 2.4gz
Ping failure from Client using 5gz-2 to Client using 5gz-1
Ping success from Client using 5gz-2 to Client using 5gz-2
Ping success from Client using 5gz-2 to Client using 2.4gz
Ping success from Client using 2.4gz to Client using 5gz-1
Ping success from Client using 2.4gz to Client using 5gz-2
Ping success from Client using 2.4gz to Client using 2.4gz
Note: Connect's fail in a similar fashion.
All three devices are Linux based devices.
I am currently considering disabling one of the 5gz radios on my RT-AC3200 router.