If an audit report just told you your site has a hreflang self referencing problem, your first instinct is probably to open the page source, find the line where a page points back to its own URL,...
If a checker just flagged your site for a missing hreflang x-default value, don't panic. It's the single most common hreflang finding there is, and in most cases it's a warning, not a broken page....
Hreflang is one line per language on every page, and most of what goes wrong is the tags contradicting each other. The five rules, the codes that fail, and the canonical mistake that cancels the whole cluster.