There was a known issue with stiction of Z-axis of the NXP accelerometers MMA845x and MMA865x families, but this issue was corrected in 2013, so, I recommend you to check the marking of the accelerometers and make sure they are recently manufactured devices.
Yes, this stiction issue affects the vibration sensor and cause it to export incorrect values, specifically, you can see data for X and Y axis correct, but Z axis is not moving at all, doesn’t matter if you move the accelerometer.
For parts that used to have this stiction issue, there was a “recovery” method, or a way to make sure the problem is caused by a mechanical issue of the internal MEMs structure, this is done by slight tap on the part. Remember that the problem was that the MEMS structure inside the accelerometer gets stuck, so the workaround was to tap it/hit it with your finger on top of the accelerometer to un-stuck it.
There seems to be no new reports of this issue for the MMA865x family in our technical support system tool. In case you find that these are recent manufactured devices and are soldered according to the NXP recommendations, then a CQC failure analysis with the NXP Quality engineers would be recommended. You would need to contact your distributor to start a CQC (failure analysis) process.