Game Pass Ultimate subscription expired today. Is it possible to get a 12-month...

It sounds like you have Xbox Live Ultimate, then purchasing Game Pass Core (Xbox Live Gold) for 12 months, which basically converted to ultimate for 4 months. Which is exactly what it is meant to do. If you wanted the 12 months, then you should have waited for Game Pass Ultimate to Expire before Redeeming the Game Pass Core. There is nothing that you can do. If you cancel Game Pass Ultimate, you will lose Your subscription, including the Game Pass Core you have just redeemed. You're better off waiting until it expires before getting another subscription.

If sounds like you already had an Ultimate Game Pass subscription that was set to expire on 01/18/2020. When you add a prepaid gold or game pass code to an Ultimate subscription, they extend your subscription, but are converted to Ultimate at a different
conversion rate. 12 months of gold/game pass is equal to 4 months of Ultimate.

If you already had a Game Pass Ultimate subscription when you added the 12 months, it would convert the 12 month to Ultimate which gives you 4 months of Ultimate. The conversion is based on price. 12 months of Gold is equal in price to 4 months of Ultimate. In order to get the full 12 months, you would have needed to either cancel your subscription or wait for it to fully expire before adding the new code.

Technically you're not. Gold and Game Pass are converted into Ultimate based on price, not based on the time. The price for 4 months of Ultimate is the same price as 12 months of gold. If you want to use the full 12 months of just Gold, you would need
to cancel Ultimate or wait for your Ultimate subscription to expire and then add the 12 months.

In order to get the full 12 months, you would either need to cancel your current Game Pass Ultimate subscription or wait for it to expire and then add the Gold after it has completely ended.