There’s a common misconception that a quitclaim deed eliminates responsibility for the mortgage. It does not. There are two separate issues: who is on the title (owns the property), and who is on the loan (pays the mortgage). A quitclaim deed signed by your ex gives you their 50% title (ownership) rights to the property, but it doesn’t affect the loan.
Here’s an example: you lend John and Sue, a married couple, $100K to buy a house, and they both promise to pay you back. They divorce, and Jeff quitclaims his ownership to Sue. She now owns the house, but they are both still responsible for the loan.