Why would somebody use an underscore instead of the actual name?

Actually there is no reason, the problem is that until something is allowed, surely in some page it's used :) In itwiki https://it.wikipedia.org/wiki/Categoria:Pagine_con_errori_nell'uso_delle_note is increased a bit for this type of errors (I don't know in other wikis). We can fix them all.

Unless there are still thousand of errors hidden by caching, in itwiki all errors were fixed. For us the issue is solved.

Other wiki has this issue too. Is it intentional?

In T181812#3804641, @Aklapper wrote:

Why would somebody use an underscore instead of the actual name?

Underscore as space, not single character in name ;)

In T181812#3811735, @Wargo wrote:

Other wiki has this issue too.

That is likely, as wikis run on the same MediaWiki software. :)

Is it intentional?

We don't know...

Underscore as space, not single character in name ;)

Sorry, I expressed myself badly. I meant "space" instead of "actual name".

I am sure this is the reason: https://github.com/wikimedia/mediawiki-extensions-Cite/commit/351a08d1b79d7817c634597178dc35a49239a741
This is example: https://en.wikipedia.beta.wmflabs.org/wiki/NewPage1495380326

In T181812#3812487, @Wargo wrote:

I am sure this is the reason: https://github.com/wikimedia/mediawiki-extensions-Cite/commit/351a08d1b79d7817c634597178dc35a49239a741

CC'ing @MaxSem and @Legoktm mentioned in that commit. (Though I'd personally decline this task.)

In T181812#3812505, @Aklapper wrote:

CC'ing @MaxSem and @Legoktm mentioned in that commit. (Though I'd personally decline this task.)

I would be inclined to decline this task as well. The original IDs working was likely an unintentional side effect and shouldn't IMO be supported.