L2 Authentication fail through is basically to peform let`s say both MAC authentication and 802.1x authentication.
Say for example, when MAC auth fails, enable the L2-auth-fail-through to do the 802.1x auth.
See below info from User-guide.
This config is not going to be applicable just for MAC auth /Captive portal.
You can open up a support case where TAC could try to replicate your config on 6.1.3.10 to see if the same issue occurs.
Thank you