Just spun up a new WP instance (5.8.3) and run in to this problem. I go to create a page with title "Testing", and then go to create a post with same title. Both get same slug, and all links that are supposed to lead to the post lead to the page instead, both back and frontend. For example if I search for "Testing" on the frontend using the search field of the theme, it finds both the post and the page, but both link to the page.
I thought that wp took care of slug collisions by adding numbers at the end of slugs that would otherwise collide? Or did I dream this?
Theme: Twenty Twenty-One
Permalinks: Post name