What is usually done is that a stiffener is added to the area of the connector. The stiffener is usually FR4 (with no copper) glued/epoxied to the flex on the backside of the connector.
However, maybe the application results in minimal bending without stiffeners. What counts as acceptable is rather dependent on the particular parts, the application, and the required reliability and lifetime, so it’s not easy to quantify.
Read More: Best Practices of Rigid Flex PCB Design