Typically when you go to reputation league, the url looks something like https://stackexchange.com/leagues/{id}/{site}
or https://stackexchange.com/leagues/week/{id}/{site}
, etc. The link contains some numerical value followed by site name, for example https://stackexchange.com/leagues/1/stackoverflow
.
Since those numerical ids of sites can be changed, when posting a link to some information league, the links of the form https://stackexchange.com/leagues/{site}
might be preferable. And they indeed work (you get redirected to a longer url including the "id"), some examples: Stack Overflow, Super User, Arqade, English, Ask Different, Server Fault,... However, the shorter version does not work for MathOverflow - you need to you a longer url which includes the numerical value - which is currently 504.
Question. Why the "shorter" link does not work on some sites? Is MathOverflow the only site where the shorter link does not work?