I think I figured it out, and @PrincipalTinker is spot on.
Because you have two use years, you have two memberships. The other reason for having multiple memberships is different names on the deeds.
And because it’s two memberships the following applies:
Subject to one exception: (a) points in one use year cannot be combined with those in another use year to make a single reservation, and (b) a reservation made with one use year’s points cannot be modified in any way by using points from another use year.
The only exception is to transfer points from one use year to the other, which, if done, allows the points from both use years to be used to make or modify a single reservation. Nevertheless, only current use year points can be transferred, not banked or borrowed points.
If you do that you might be able to book online.
The discrepancy between what some Liners have done and this is the multiple memberships. If you have multiple contracts with the same use year and therefore under a single membership, you can use all the points as one big pot and pick which contract you want the points to come from.
For example, you can use all banked points first from all the contracts, and then use current points for one specific one. Or you could use all points from one contract.
I need to look and see if I can find the info about the two ways to book though. But that only applies if you have a single membership, so not applicable to @ryan1.