From: Bay Area, California
Here's my guess:
It became independent. But Spain had troops there and since there was no garrison, Spain conquered it immediately.
Let me recount why I made that assumption. I was Sweden, at War with Denmark and Russia. Russia Surrendered and as part of the peace, I made them liberate every Danish Province. I still had an army in one of them and the same turn Denmark became liberated, I declared war on Denmark. I saw that I conquered the province and none of their provinces except the capital had a garrison.
However next turn, inexplicably, the war ended, the province reverted and all provinces had garrisons. I tried 3 or 4 times to redeclare war on Denmark but it kept ending a turn or two later. That bugged me, and I think it's a bug that has been noted.