Some things that come to mind to try:
Does this happen for all post types or just post or just custom post types? Are they hierarchical posts?
You could read through the wp_unique_post_slug function (https://developer.wordpress.org/reference/functions/wp_unique_post_slug/) to see what might be triggering the suffix.
Take a look through your database to see if there is any lingering post_meta that might be causing the duplicates (meta key = _wp_desired_post_slug).
See if there are any hooks that alter wp_unique_post_slug.
If you have a post with the slug 201555555-2 and you add a new post with the slug 201555555 does it get a -3? Is the -2 only added to number -only post titles or every post title even with only letters?