Hi there!
Aside from the above about Xbox game bar, if you remember installing any third party app. I suggest checking them as well as sometimes some apps has a shortcut key to start a recording, clipping, or screenshot. You can also check inside Fortnite Settings → Keyboard Controls:
- Make sure C is still bound to Crouch
- Make sure no secondary bind conflicts with recording
You can also try disabling some startup apps by pressing CTRL + SHIFT + ESC to open the task manager. Check each apps you have and try disabling them 1 by 1 and restart your device before playing Fortnite again then test if "C" key is still triggering the clip function.
Please note that this is a public forum, I will be responding you in short period of time and regret for a delay in response. As I am merely a fellow user trying to provide insight and information that may be helpful to others. Thanks!