Several (more than 100) players appear simultaneously on the NA and EU achievement leaderboards. Picking one example completely at random, Calixtus.8617 appears both on https://leaderboards.guildwars2.com/en/na/achievements/world/Ehmry%20Bay
and on https://leaderboards.guildwars2.com/en/eu/achievements/world/Piken%20Square.
(If I were to hazard a guess, I suspect that the data is being cached somewhere between the sever(s) responsible for maintaining the rankings and the server(s) responsible for delivering the web page and there is no mechanism in place to flush the cache entries when a player transfers from one region to another. As the saying goes, there’s 2 hard things about computer programming: cache invalidation, naming things, and off by one errors
).
On a somewhat related issue, I can routinely invoke 500 errors on the leaderboard servers by trying to simultaneously download 4 or so different pages simultaneously, so I think you have other issues there as well.