I’m having the same issue with Yoast SEO 15.8 and Classic Editor 1.6 installed. I’m getting the “Changes that you made may not be saved.” popup every time I try to navigate away from a post whether I’ve made any changes or not.
Deactivating either plugin makes it stop so it must be the combination of the two. This just started happening with a recent update of Yoast SEO. Considering that’s a pretty common combination, I’d like to find a solution to avoid the annoyance.
Were you able to find a solution @nickylew?