Capabilities and mapping required for a role to be able to edit other’s posts of a custom type, BUT only be able to edit their own blog posts

Turns out this plugin was the cause of the issue:

View Own Media Posts Only

https://wordpress.org/plugins/view-own-posts-media-only/

One disabled, custom capabilities worked as you would expect them to.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)