Hi bdormael,
Our engineers evaluated the issue.
The main problem with this project is RAM, due to the huge amount of SpinFX (35! incl 2 in send) in combination with stretch.
On iPad 7 and one of our iPhoneX it is not possible to load the project in the first place.
With some of the spinFx removed the project loads and the mixdown works as expected also.
Please give it a try to reduce the number of SpinFX effects.
Using multiple SpinFX per track may not be necessary. Using SpinFX as send effect may not be necessary.
Hope that helps!
Best,
Lars