CSS disappears

I would suggest speaking with your host. Tell them of the issue that the Additional CSS is removed after save.
Ask:

a) if there are any server side caches that could interfere with this
b) if there are any errors in the database or records of the custom_css post type being edited or changed.

I'll do that....
In the meantime - do you have a plugin to recommend to add css?
This should be as simple as changing the size of heading text. WP should be ashamed....

You can try this plugin:

https://wordpress.org/plugins/simple-css/

Thanks.

You're welcome

I wanted to tell you that I finally solved the problem (after more work and discussing with my host).
I deleted the Generatepress theme and re-installed it.
Problem fixed.

Glad to hear you got it working!