I created a shirt in Marvelous Designer, imported it into CC5, edited it as clothing, and saved it. When I wear it on my character again, why does the seam break? It was fine when I first imported it. What could be the problem?
The seams to be welded. Right now your shirt is separate panels of cloth. MD export should have an option to weld the vertices of the seams.
You can also weld the vertices in CC if you need to. See here:
https://manual.reallusion.com/Character-Creator-5/Content/ENU/5.0/08-Creating-Custom-Assets/Welding_Vertices.htm
Make sure the shirt is in it’s default shape (so not distorted at all) when you weld the vertices.