Sorry, I mistyped my repro steps. This is the link that doesn't work:

https://test.wikipedia.org/w/index.php?title=Special:MyLanguage/CTTSBS

Correct steps:

Create page "Complicated title that should be shortened"
Create page "CTTSBS" with content: "#redirect [[Complicated title that should be shortened]]"
Visit index.php?title=Special:MyLanguage/CTTSBS

I should note that it does not matter if the redirect is [[Special:MyLanguage/]] or just [[]]

On test.wp:

• ✔ [[A]] redirects to [[B]]
• ✔ [[Special:MyLanguage/A]] redirects to [[B]]
• [[CTTSBS]] should redirect to [[Special:MyLanguage/Complicated title that should be shortened]] (T164357)
• [[Special:MyLanguage/CTTSBS]] should redirect to [[Special:MyLanguage/Complicated title that should be shortened]] (this bug)

So, this is a kind of 3-steps redirection, I think this bug may have two different origins:

• redirecting towards an already bugged page: so T164357 is a blocker ;
• or this is same issue than visiting Special:MyLanguage/Special:MyLanguage/A.

In T196861#4270695, @Danielhats wrote:

This is the link that doesn't work:

https://test.wikipedia.org/w/index.php?title=Special:MyLanguage/CTTSBS

It (sort of) works for me: it doesn’t redirect to the target page (T164357), but it’s not blanked out, it just shows the same content as if I’ve appended &redirect=no to the URL. If I link to a redirect that doesn’t itself point to Special:MyLanguage (e.g. https://meta.wikimedia.org/w/index.php?title=Special:MyLanguage/VRT&uselang=de), it works as expected. Am I just lucky, or has this bug been inadvertently fixed?

In T196861#7561678, @Tacsipacsi wrote:

In T196861#4270695, @Danielhats wrote:

This is the link that doesn't work:

https://test.wikipedia.org/w/index.php?title=Special:MyLanguage/CTTSBS

It (sort of) works for me: it doesn’t redirect to the target page (T164357), but it’s not blanked out, it just shows the same content as if I’ve appended &redirect=no to the URL. If I link to a redirect that doesn’t itself point to Special:MyLanguage (e.g. https://meta.wikimedia.org/w/index.php?title=Special:MyLanguage/VRT&uselang=de), it works as expected. Am I just lucky, or has this bug been inadvertently fixed?

The response is empty, because MediaWiki sets a url redirect to https://test.wikipedia.org/wiki/Special:MyLanguage/Complicated_title_that_should_be_shortened - but that page itself does not has content, because it would redirect as well, but that is not done when $wgHideIdentifiableRedirects is true. The used soft redirect does not work for that redirect-redirect. But when the setting is not in affect it gives a redirect loop and the browser times out.
Maybe all special pages implement RedirectSpecialPage should be invalid redirect targets (like Special:MyPage/MyTalk already are). Or Special:MyLanguage implements some special for this, but must look for loops as well.

Maybe you should define what you mean by “the page returned is completely blank” and “[t]he response is empty”. I interpret these terms as referring to the White Screen of Death, but I don’t experience that. I get a completely normal 200 OK response with valid content that looks like this:

Yes, the behavior has changed: we previously got a fully blanked page (no content, even wiki’s header, menu and footer).

The behavior for Special:MyLanguage/Special:MyLanguage/Redir2 has changed too: it now leads to home page (according to my 2020 comment, it returned a blank page too).

In T196861#7565151, @Tacsipacsi wrote:

Maybe you should define what you mean by “the page returned is completely blank” and “[t]he response is empty”. I interpret these terms as referring to the White Screen of Death, but I don’t experience that. I get a completely normal 200 OK response with valid content that looks like this:

Try it logged out. The page has a "resource size" of 0 with a 200 OK and does not redirect.

This is essentially the same issue as T164357

Can you give an explanation why it is the same issue?