It's possible that your tree was propagated sexually and dominant genes from one of it's parents is producing the unexpected flower color. M. x 'Elizabeth' is a selected seedling of a cross between M. acuminata (Yellow cucumbertree), a native species and M. denudata (Yulan magnolia), a native of China, which usually has white flowers, but can produce redish/purple ones, like this:

Mine produced mostly yellowish/creamy/white flowers when young and only after 25 years or so, did it produce true yellow ones.

M. x 'Elizabeth' was a patented plant and many growers propagated it sexually to avoid the royalty fees. The patent expired in the late 1990s, but I guess old habits are difficult to change! Why change a method that worked for many years.

There is no way to determine if the flower color will revert to the desired yellow.
I have a yellow-flowering Camellia, whose blooms emerge yellow, but change to white before they fall off. I'm certainly not pleased about that!
Good luck.
Rb

Thanks RB; that must be it. Funny, though, we have had it for five years and last year was the first year that it bloomed. I could absolutely swear that it bloomed in the usual yellow! Very, very strange!