usually they merge servers from the same clusters or so, however i think iv seen the case of server 1000+ merged with under 200 (not sure how or why) but in most cases they merge servers close to each other, sometimes you get merged with heavy whale driven server and its painful yeah...

i dont know if is a bug or is intended but the reward is given only once lifetime and is sort of "random" meaning some people get it and some dont, the people that dont get it they might get it next merge and the people who got it in 1st merge dont get it in 2nd... same with the 3rd with people that didnt got in in neither 1st or 2nd, and i'd assume that is for how many times the server is merged (my personal record is 4 merges on same server i think)