I sure hope not. It's kind of sad seeing everyone else getting theirs delivered now and for some reason mine is in May!
The yields on the 64-core GPU Ultras are likely pretty low. From what I understand from folks who know a lot more about chip fab than I do, the most likely way Apple are doing it is printing the interposer fabric between two adjacent M1 Max chips on the die, to create the Ultra.
So, in order to fab the 64-GPU core Ultra, you need two "perfect" chips to occur side by side. Whereas for the 48-core version, as long as neither of the adjacent chips have more than 12 defective cores each, you can link those up and you have the lower-end Ultra. There's even some debate about whether Apple could link, say, a chip with 32 functional cores with one with only 16 functional core - or any combination that adds up to 48, really - to deliver the "base" Ultra.
For the Max, you just need either some (24-core) or all (32-core) of the cores to print without error, without the added probability of being immediately next to a similarly good chip.