When I joined AAISP I looked up their blog about the B10A and the firmware, as that was what they were going to sell me. (They previously supplied something else, and I didn't need to take their "modem").
There were several tweaks ZyXel applied to the firmware. Some for bugs AA found, and IIRC some requests as well. If I'm right, that would explain why they wouldn't want to apply this MTU mod, is it could well have meant a lot of work to do it on the AA version, when AA are no longer buying it.
Just a guess, but a feasible one.
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