The System is suppose to be setup so that soonest expiring Credits are used first. Whether they are your Credits or OTU Credits. Not whether they are OTU Credits. So if the System works correctly if you later purchase OTU Credits that expire before the Credits you have in the Booking the Shuffle (if working LOL) should take care of the problem.
The only other factor is that WM Credits are suppose to be used before WM+A or WMTS if they have have the same expiration Date.
Under the Old System (now gone for so many years but still missed) the Credits were listed as WM, WM+A, or WMTS