Thinking about it some more, I’d not go against HTML spec – I personally have never needed to use nested links in title, so can’t say what’s would be a common use case, but I’d say the implicit-closing-of-anchors is far more UX-unfriendly than disallowing nested links to begin with.
E.g., writing
[the [aquila](url) rift](url2)
will IMO leave user more confused with the output due to the prematurely closed link, than if we just disallowed the nested link altogether, which will make it obvious what’s happening from first sight.
It’ll also teach users early on how to properly write links.
This argument obviously stands on the fact that markdown is mainly used for HTML output, where nested links don’t make sense. Is there even an output format where it makes sense to have nested links?