Special:RelatedChanges is an alias for Special:Recentchangeslinked. Until recently, subpages of RelatedChanges redirected to Special:Recentchangeslinked, so that, for example, Special:RelatedChanges/foo redirected to Special:Recentchangeslinked/foo. Currently it redirects just to Special:Recentchangeslinked.
Hi @Yair_rand, thanks for taking the time to report this!
This might be a duplicate of T181100: [wmf.8 - regression] Related changes: Saved filters set as default won't include the page as part of a query?
If you agree, feel free to merge via → Close As Duplicate. Thanks!
Seems to be fixed now. https://en.wikipedia.org/wiki/Special:RelatedChanges/foo now redirects to https://en.wikipedia.org/wiki/Special:RecentChangesLinked/foo.
Feel free to re-open if I misunderstood.