I remember we had this exact problem on nowiki as well. I tried a lot of things, but I don't remember if I did anything to solve it – but it is working now. The last thing I did may be relevant to try for iswiki too, it could be that that was what solved it. Or it could be that the parsing is delayed for some reason? Either way, worth a try at least.

In T284277#7133637, @jhsoby wrote:

I remember we had this exact problem on nowiki as well. I tried a lot of things, but I don't remember if I did anything to solve it

Could be T250807: Dates in citations on Special:CiteThisPage are in <p> and hence create unneeded and ugly linebreaks and T193267: The <citation> tag is not being parsed on Special:CiteThisPage on some projects?

In T284277#7150108, @Aklapper wrote:

In T284277#7133637, @jhsoby wrote:

I remember we had this exact problem on nowiki as well. I tried a lot of things, but I don't remember if I did anything to solve it

Could be T250807: Dates in citations on Special:CiteThisPage are in <p> and hence create unneeded and ugly linebreaks and T193267: The <citation> tag is not being parsed on Special:CiteThisPage on some projects?

Ah, yes, definitely.

Anyways, it seems like the problem described by @Snaevar for this bug no longer exists – the page output looks correct to me, even though the relevant MediaWiki message hasn't been edited at all since they reported the bug. So I think there's just some delay in the <citation> tags being parsed correctly? Might be worth a separate bug, but I think this one can be closed.