2023 Pension Splitting Optimizer Not Calculating Like Prior Years

I've been doing one couple's taxes for several years now and the pension optimizer calculation has always had a curve in the calculated Net Combined refund column's amount - as the Split-pension amount increases, the refund climbs until it hits an optimal amount of refund, after which, increases in the split actually cause a decrease in combined refund. Here's an example of 2022's calc (hopefully it comes through):

This behaviour has been consistent over 2020-2022's returns.
This year's (2023) behaves in a straight linear fashion - only the maximum allowable amount seems to calculate the largest refund. I suspect something has been changed, because this is counter-intuitive; as more income is transferred to the spouse, their income is rising and the amount of tax payable by them is rising also. But an increase in spousal income shuts down some of the transfer of credits (one spouse is disabled) so the maximal benefit isn't realized by the higher income spouse.
Has anyone else noticed this happening in 2023?