Accelerometer MMA8451 issue
We have talked to NXP.
They had issues regarding the MMA8451 ealier. Back in 2012 they implemented more quality testing at their factories to help with the issue of a "sticky z-axis".

We have checked our components, which are of newer dates (2016 / 2017). But we are still convinced that we see issues related to this.

NXP also says that this can happen if the forces are too big on the accelerometer.

We are going to change the accelerometer on our product.

Thanks for your suggestions.