It isn’t a bug, races are different, some get trapped behind objects and Gnomes swim in shallow water. Getting knocked up onto a ledge and a boss evading isn’t a bug and it isn’t going to be fixed.
Happened to us this week, don’t know if it was a Gnome that caused the evade but our Gnome tank wasn’t being hit until long after the boss reset back to full HP:
https://classic.warcraftlogs.com/reports/F1grbVkv4mWG9KxA#fight=17
Had to kill it twice, I didn’t come to the bug report forum to file a false bug report though.
Not every quirky little thing in this game is a bug that needs fixing.