If you are set to seek battle, it should result in a battle in the province. However you will ofc need to WIN said battle. If you lose the engagement, the enemy siege will continue.
As to why the union AI takes the forts so quickly is usually a combination of things;
-they normally have larger forces there (every brigade fires at the besieger).
-they normally tend to have added things like engineers to their brigades.
-they often use aggressive settings in their sieges to break it faster.
I hardly keep forts as the CSA, a few select ones, and for those I buy upgrades, both at fort level, fort guns and brigade guns. Normally I take the brigades in the forts and add them as regular brigades to my armies where they make more use of themselves (well, eventually, as they start out very green).
"Hun skal torpederes!" - Birger Eriksen
("She is to be torpedoed!")