Awwww.... that's too bad. I'm lucky that my XL is still functioning well. Yup, encountered people attacking while someone is mounted. Being a purist gunner, I can see if someone is mounting the monster. Sometimes other blademasters won't see it because they are too close to the monster and they didn't know that its being mounted. Then there's a few that will still attack it and causes the hunter to dismount.
I played with randoms ever since mh3u on the wiiU. They made online hard in mh4u due to the lack of microphone use and limited chat messages. Sometimes there are those mean randoms... I entered a room one time then one of the players messaged "Oh my a gunner" (the one who said it carted twice in the quest). And a lot of times they just kick me out without seeing me play.XD so yeah, nothing beats a local hunting session.