Transient drop in accelerometer gravity reading when in motion
It could be inherent to the sensor technology used. If they use spring-loaded mass for sensing, you might expect that the force vector is offset to a certain extent during lateral movement. If you assume, that total length of spring is limited, then it gets clipped on Z-axis to the remaining length allowed for movement.
E.g. sum of forces E is constant 1 (or something slightly above it). If you have only gravity working on it, it &quot;consumes&quot; most of it. If you introduce lateral movement on +X axis with magnitude &gt; 1, then Z drops to 0, for it's &quot;overpowered&quot;. If you introduce movement on +X axis with magnitude less than 1 (say, 0.5), then you get much lower dip in Z, but it's still present. Which seems to be the case.
Damn, it's difficult to explain, but I have it somewhere in my headspace :) This image should illustrate it somewhat:

except here springs are ideal, while in reality they have stiffness and elasticity coefficients, which limit the &quot;ceiling&quot;
I haven't used this particular STM sensor, but others by them (LIS2/3) don't seem to exhibit such properties.