How to change URL Custom Page?

You’re using the reserved public query variable name as your custom one.

It can e.g. affect the canonical redirect by setting page_id and name for a different page.

Change it to something else to avoid possible name collision, like kenan_video_slug:

add_rewrite_rule(
    '^watch/([^/]*)$',
    'index.php?pagename=watch&kenan_video_slug=$matches[1]',
    'top'
);

Note that your rewrite will override the content pagination for the watch page.

Remember to flush the rewrite rules.