Yellow
I agree that having a reward for test server can make people play on that server just for the freebie, which would be counterproductive, but there could be a better way to distribute the reward. Like if they want feedback, make the player do a mandatory survey or something so they can get feedback on what they've specifically changed. but I suppose this could go badly and lead to people just filling in random information on the survey just to get the reward but I'm sure there is a way to weed these players out, if a player takes advantage of the system and gives unrelated answers or bad answers, prevent them from playing on the test server in the future or something. I'm not really sure how to deal with the trolls and freebie seekers.
I just think testing stuff and giving feedback can take a lot of time and effort and getting nothing for it does feel kind of bad. but how else do you incentivize players to give feedback?