Hi Perene,
after i wrote my post i opened a support case. Here is their answer:
"This is expected behavior. The iPad will only allow you to configure auto-lock settings to the exact value as configured on SM or less.
This is Apple's way of avoiding confusing the user. If the user is allowed to set a higher value and the user chooses this higher value, he will get the impression that his configured setting will work, but it will not, as only the most restrictive settings between SM and what is configured locally will be applied. That is why Apple proactively disable higher value options.
If you want to have all options available locally on the device, set auto-lock to never on SM"
I realized that the only way you can change it, is to do it on the device manually. Otherwise you need to set auto-lock to never as they explained above.
Best regards