Admin page title not visible
The issue happens when you have selected a dark content color background from the Customizer. That color could have very similar values with the color WordPress sets when a post title is being entered or when the admin post title input is focused. To counteract the issue, the plugin Add Admin CSS can be used to add extra styles on the page and post-editing admin title input field so that the color is set to a different color in both cases.
Just go to Appearance -> Admin CSS and write your CSS in the Admin CSS text area.
.editor-post-title__block.is-selected .editor-post-title__input, .editor-post-title__block .editor-post-title__input:focus { color: #FFF; }
In the above example, I have set the edited and focused color to white
.