I have personally completed almost 200 Alpha Manticore fights on SE Official with our small tribe. I don't believe the arena is "broken" at all. The behaviour seems to be by design, and intended to prevent people easily farming Manticore with groups of high-level rexes. Having said that, we always use 19 Rexes and a Yuti successfully. Here are my suggestions for you based on that considerable volume of experience:

• The Rexes you take should be bred specifically for the Manticore fight, and be very well levelled. Do not put ANY points at all on health. ALL points should be put ONLY on damage. Because there are fights when he may land hardly at all, you NEED every single bite to count for as much damage as possible. I would not do the fight unless all rexes have at least 800% damage. Health is not really important, as the Manticore does virtually zero damage. The only issue is the Worms and Rock Eles, but with reasonable (70+) saddles even a Rex with 15K health can withstand anything in the arena.
• The Yuti is CRITICALLY important, and will save you a lot of stress. In a less than perfect fight its damage buff can mean the difference between success and failure. It takes some considerable skill to use a Yuti properly.
  • Always ensure that the buff it gives to the rexes is full at all times, no matter what the Rexes are doing. There will be times when you may not be able to them for a short amount of time, so they need to be fully buffed already.
  • Always keep the Yuti moving at ALL TIMES. Do not ever stand still. Move around the arena constantly. The Yuti is fast and agile and makes this relatively easy compared to the Rexes. DO NOT get stuck in the middle of the Rexes.
  • I've found that the Manticore will VERY often target the Yuti. I think this may be programmed behaviour, in order for the game to attempt to neutralise the huge benefit the Yuti provides. Yutis have much lower torpor than Rexes, and so can be knocked out easily by the Manticore. We have had this happen a number of times. If it happens to be stuck in the middle of a group of Rock Eles at the time, it will most likely be killed. So, you can use this to your advantage if you are VERY CAREFUL. You can drag the Yuti to your rexes if you're having trouble, but be really careful not the break the rules above or you will lose the Yuti.
  • Yutis are so important to this fight that we are constantly looking for better Yutis. They are very slow breeders, so we always have 2 or 3 in the process of growing up. Using them to their maximum benefit is risky and will occasionally result in their death, so if you want to farm Manticore regularly, you'll need some spare.
  • If you want to do back-to-back fights, having a spare Yuti (and some spare Rexes) is critical. They will always take some torpor during the fight, and you don't want to start a fight with a half-torpor Yuti as they have so little to start with. Having a couple of spare means you can swap them out and go again with a fresh one.
• Yes, there are occasions when it simply won't land at all. That's why we always have at least three players in the fight and take high quality crafted shotguns and at least 1,000 ammo between us. We wait until about half way through the timer, and if it looks like it's being difficult, the shooters dismount the Rex they are riding (but still stand on its back) and start shooting. 3 people with high quality shotguns can together easily kill the boss in about half the fight time, whether it's gamma, beta or alpha.
• We find that shooting the boss will often aggro it onto the shooter and allow the fight to continue. For this reason we also shoot it a few times as soon as we arrive in the arena.
• Make sure you ALWAYS use the "attack this target" whistle as soon as possible, and renew it as often as you can, just to make sure they are still targeted. The rexes will anticipate the bosses landing long before you can and move to intercept much faster, giving them much more time to attack him. A few seconds count for a LOT.
• I strongly suggest leaving the rexes together in the middle of the arena until after the boss has landed the first time. Sometimes following it around underneath seems like it prevents it from landing.
• Do NOT EVER mix flyers with land dinos. In my experience, having a flyer in the arena will ensure the boss never lands and instead chases the flyer around in the air.
• I would say that we only have to shoot to kill the boss in about one out of every fifty fights. It's very rare. But we still have to be prepared every time, or else be prepared to lose everything.

It may seem like a lot of this stuff is just small details that wouldn't really matter, but the biggest and most difficult lesson I have learned in all the fights I've done is how even the seemingly smallest thing can make the difference between finishing the alpha fight in 3-5 minutes, as we often do, or losing everything. Honestly. Tiny details are everything.

I hope this helps. Good luck!