Help, Bad Result Of Clothoid Curve

[Starbirth2002]

Thank you for your sharing. I've encountered an issue where. under most circumstances, I achieve satisfactory results. However, there are cases, as shown in the image, where it appears there is a miscalculation in the length of the Clothoid curve. My mathematics is quite average, making it difficult for me to understand functions like C(), S(), and SolveBrent().
When this issue with the image occurs,
Circle1: center(900, 500), r:300
Circle2: center(800, 450), r:500