IBM APAR/PTF UI79543 supported "Enable Dynamic Instance Names" with a default set to "Y" .
Looking at the DOC HOLD for UI79543, part of it states:
Dynamic Instance Names must be disabled when the system security manager does
not support the RACROUTE request used by MFA to determine the defined factor instance names.
Setting "Enable Dynamic Instance Names" to "N" will circumvent the problem.
ACF2 has support for the RACROUTE REQUEST=EXTRACT,TYPE=EXTRACTN call that is used by IBM MFA 2.5
It is apar LT05014. This is a test apar and includes many other MFA related enhancements.
We are currently going through final testing in-house.
if you are willing to apply and test under these circumstances I can send the apar and related documentation to you.