Thanks for writing. If your plant prefers part sun, that usually means it does best in a spot with morning sun and afternoon shade. If yours is getting afternoon sun, it'll probably help to move it to a spot where it's protected from the hot afternoon sun.
Also: If you just purchased it from a greenhouse, it may need to get adjusted to direct sun --- if this is the case, the new leaves shouldn't be affected as much by the sun.
---Justin, Senior Garden Editor, BHG.com