I have an Apple Watch Series 6 too, but this has never happened to my iPhone, so this may be a small bug inside yours. Make sure your iPhone is updated to the latest version first (latest versions of a software usually solves certain bugs).
Follow Step 4 in this article: If an app unexpectedly quits, stops responding, or won't open. If this doesn't work, contact Apple Support online (with the link inside the article), and see how they can help you.
After this, update your Apple Watch too, because I am unsure whether if this problem is caused by pairing your Apple Watch, or a native problem caused by the phone itself. But updating your device is always a good option to reduce the chances of bugs happening.
Totally optional: Once you are done, you can visit this page: Product Feedback, just so that Apple understands your problem and can make changes in future updates so that this doesn't happen to other Apple customers.