The query /users?limit=1&facets=patronGroup:50 returns a 500 error with the response body, "Internal Server Error, Please contact System Administrator or try again" in mod-users-15.7.0-SNAPSHOT.84. Using a smaller value, e.g. patronGroup:5 or patronGroup:1 also fails. Faceted queries against mod-users-15.7.0-SNAPSHOT.82 are successful, so it seems this bug was introduced in SNAPSHOT.83 or SNAPSHOT.84.
Practically, this means the Settings > Users > Patron groups page displays an Okapi error and integration tests that visit this page fail.