time to add yoghurt

Yogurt is used in Indian and Pakistani dishes, but some people also add yogurt to curries at home.

Yogurt contains the same proteins that curdle milk and lean cream, and when heated with sour ingredients, it separates into curds and whey.

Separation in yogurt is caused by acidic ingredients such as tomatoes, vinegar, lemon juice, and fruit. Also, it will curdle at high temperatures, so yogurt should be added at the finishing stage to prevent separation.
In other words, add it when it's cooling, not when it's simmering.
Raw milk hardens when making cheese because the milk components and casein contained in the milk protein harden.
The casein is coagulated by adding either rennin (a proteolytic enzyme extracted from the calf's abomasum), salt, or vinegar. Heating also changes the structure of casein, causing it to solidify. (One of the reasons fried eggs harden)