I agree that it shouldn't.
I spent years as my younger self bouncing around in M113s and M577s and because of their aluminum hulls they're pretty light, bounce and vibrate and are loud. I can't ever recall anyone getting sick in them even when buttoned down in the back. Mind you fifty years later I do have tinnitus to which they no doubt contributed - but at the time . . .
You can run tests on the physical aspects of vehicles and my understanding is that AJAX had been tested extensively and whatever was wrong with them initially had been worked out. The basic ASCOD model has been in service with Austria and Spain for decades. It's not like its experimental.
The problem now is what do you do with almost 600 of these things which were supposed to be the stars of modern cavalry warfare. These aren't the type of injuries that can be easily confirmed or analyzed. My guess is that regardless of what is done, there will now always be some folks complaining of these symptoms.
The simple fact is that operating heavy machinery, whether military or construction, comes with high levels of vibration, noise and erratic movement. Those can cause headaches, disorientation and nausea (especially when buttoned up).
I know that these days it's not on to say "its all in their heads," . . . but. Clearly some additional independent testing is necessary - such as comparing the physical conditions in AJAX to those in older generations of tracked vehicles and then comparing the identified medical conditions of the various users. Better ear protection is an easy solution. Transferring folks who get nauseous to a different branch/workspace may be necessary as well if a "fixable" issue on AJAX can't be identified. My guess is after years of dealing with this, there are no longer any "fixable" issues to address with the vehicle. It may need to turn to removing the folks who can't handle that work.
Rock. Hard Place.