It seems counter-intuitive for the help buttons that can be added to certain elements (e.g on FieldLayout) to have a positive tab index, allowing users to tab to them. For the best UX, it should be expected that pressing tab in a form will tab to the next field, not to the help button.The help popup buttons are inconvenient when you want to view the help text (they require clicking a small target that is often way off to the side), and they are inconvenient when you don't (they get focussed when tabbing through a form).
Examples where people have been complaining about them:
* {T181854}
* https://commons.wikimedia.org/wiki/Commons:Structured_data/Get_involved/Feedback_requests/Multilingual_Captions#Version_2:_Caption_field_added;_help_text_visible (quietly changes the design from the help buttons to always visible help text)