APEX 24.1
This has been reported before without eliciting a response, but as it's now also having its unwanted effect on us, I thought I'd resurface it in the hope of getting an explanation.
It is a simple issue. In a region header slot (Copy/Edit etc) buttons with the Text template and Default margin settings are displayed as expected with the default gap between them, while buttons with the Text with Icon and Icon templates are not.

This is due to this rule at line 11721 of Core.css
:
.t-Region-headerItems--buttons .t-Button--icon + .t-Button--icon {
--a-button-gap-x: 0;
}
Is this actually intentional? If so, what is its purpose? If Icon buttons are to be displayed toolbar-style then shouldn't it be left to the discretion of the developer to use the Button Set template options which—as shown—looks better anyway?
For now we are deploying a simple workaround in a stylesheet containing fixes for various bugs and dubious UT features:
.t-Region-headerItems--buttons .t-Button--icon + .t-Button--icon {
--a-button-gap-x: unset;
}
@shakeebrahman-oracle @tim-kimberl-oracle @paige-hanssen-apex-oracle