"The heading level is adjusted according to the enclosing context. This applies whether the heading was initially written in the documentation comment as an ATX-style heading (using a prefix of # characters to indicate the level) or as a Setext-style heading (using underlining with = or - to indicate the level).
For example, a level 1 heading in the documentation comment for a module, package, or class is rendered as a level 2 heading in the generated page, while a level 1 heading in the documentation comment for a field, constructor, or method is rendered as a level 4 heading in the generated page.
This adjustment applies only to Markdown headings, not to any direct use of HTML headings."
OTOH, JDT/UI could also argue that javadoc view and hovers don't generate a full documentation page, so there are no enclosing sections and hence section headings don't need to be fitted into such overall structure.
Follow-up after #1523: according to https://openjdk.org/jeps/467#API-and-implementation headings inside markdown comments should potentially be adjusted depending on context:
OTOH, JDT/UI could also argue that javadoc view and hovers don't generate a full documentation page, so there are no enclosing sections and hence section headings don't need to be fitted into such overall structure.