How would one know if there are the same number of pages in each blog?
Your french blog may have 12 pages displayed depending on its parameters, and English could have 6.
Therefore the language filter (and the module...) would need a special setting, only for com_content (and blog or featured) where we would have to check $this->pagination->limitstart, then check if it is similar in the asssociated menu item in the other language (Which may NOT be a blog...), etc.

Search Engines are very happy with a permanent url, not one you get via the limitstart.

@infograf768 Thanks for your comment. I understand the limitstart issue and I can see that that's a problem. Maybe there's no easy fix to that in Joomla.
However, allow me to respectfully disagree with your comment regarding Search Engines being very happy. Let's say you have 50 pagination pages on your blog. All of them has the autoreferential hreflang tag pointing to the first page of the blog instead of itself. So Search Engine are considering the very same page for this tag for all the pagination pages which is technically wrong: content are very different from a page to another.
Same problem arises in the language switch module where there's no actual link to an internal pagination page (besides the first one) from an alternate language: less links, less probability to be crawled.

Anyway, I'll let the Joomla team decide whether is it critical or not but that's definitely an issue to me.

Maybe a setting in the association tab would be to check "on" or "off" the following feature:

IF the associated menu link is a blog, then if there's no corresponding page in the alternate language, redirect the user to the last page of the blog in the alternate language.

ELSE redirect to the same pagination page.

Just an idea!

I think Google doesn't usually list a blog view itself, they list the individual articles within a blog.
Beside the pagination link would change over time as new content comes in which moves older content back.
So the pagination in a list doesn't matter much to search engines. Afaik they actually ignore it.

From a CMS point of view, we just can't know if the page "x" in french has the same content as page "x" in english. Because you may have more or differently ordered (or with different ACL) content in one language, the pagination can change.

What I'm wondering: What is the actual use case? Because I imagine a visitor browsing your english blog page, and then suddenly on page 5 he thinks he wants to switch to french? Doesn't sound like something I would do.

Hi @Bakual, thanks for your input. There's no special "user" use to it. SEMRush (powerful SEO tool) is reporting this duplicate hreflang tag as an error this is the original reason I am starting to enquire about this.
So it's only from a SEO standpoint that I work on the matter. I don't think it's critical because as you said, Google may not index blog view. It's just that those tags are incorrect on our site in Joomla blog view source, that's it. Maybe it's not critical but improving it can only increase relevancy for Google thus ranking for the site.

I still do understand the problem of having asymetrical menu link association in Joomla core. For us our English version is strictly identical to the French version, hence my initial question.

I still do understand the problem of having asymetrical menu link association in Joomla core. For us our English version is strictly identical to the French version, hence my initial question.

"For us" is the key here. Joomla is in this regard much more advanced than many CMS.
Symetrical is a common use indeed but J goes further.
A blog menu item can be associated to whatever menu item one needs, depending on the language.

@infograf768 so this is expected Behaviour?

Yes. Closing.