In Bitbucket Server 4.3,
BSERV-7360 was fixed and the UI was updated to wait the configured server-side merge timeout when merging a pull request and when checking if it could be merged. It appears the "canMerge" portion of that fix was almost immediately regressed, in 4.4.0. As a result, while the UI will still wait the configured merge timeout when merging a pull request, viewing a pull request can still show a "Server unreachable" popup while checking mergeability.
Because the mergeability check result is cached, so long as the pull request doesn't change, refreshing the screen after a timeout will (generally) show the correct mergeability state very quickly, since it doesn't need to perform a new merge.