As the title said, minecarts rotates back instantly when the pitch is set with commands. With the yaw it works fine. This issue is probably a duplicate, but the report I found is "Invalid" and reported years ago.

This could be fixed with a NBT tag. Making the "NoAI" tag useful for the minecarts, or giving a custom tag for the minecarts, like "NoRotation".

Please don't ignore or mark as duplicate, this is an important bug for someone, like me. If any more information needs to be provided, then please let me know in the comments and I'll do it.