Looking in my RT-AC68U the answer is there:
(...)
Max Limit: 64.
I wonder though why your mother need a MAC filter list of 64 or even more addresses, I assume she only has a few devices.
Besides, MAC filtering is not considered a safe way to keep strangers out.
The problem seems to happen just reaching that limit, the exact amount of devices imported from the old router was 64. I erased a couple, so no idea if could have worked just with 63. All of them were correctly listed in the webui though.
Well, it's the old family house and we're a big family, so there's a bunch of people gathering in Christmas and other significant dates, plus friends, etc, 64 if not that big number, nowadays with everybody carrying connected devices. But sure there are in that list still lots of old devices.
I know how easy is to bypass a mac filter, but it's just and extra layer of security and could help filtering some exposure.