Interesting: also not my expected behavior. But it looks like according to the help page, this is actually expected behavior (not a bug): https://en.wikipedia.org/wiki/Help:User_contributions#Accessing

Also, if you select the option to "Search for pages containing 1.2.3.4", you'll get the expected search results page. It seems like at some point, somebody added a feature to quickly search for contributions based on IP

Apparently it was added in 2004: rMW7c98afbdb7ae: Handy shortcut: paste in an IP address and click go, redirects to the user… Makes sense on a small wiki, I suppose. It's configurable via $wgEnableSearchContributorsByIP - maybe time to disable it for Wikimedia wikis? It's probably more common that a reader would want to search for an IP-like string than an editor relying on this feature to check an IP's contributions, when we have navpopups etc. these days. (cc @tstarling)