Volumetric Soil Moisture Level 4 behavior in ERA5

Hi there,

I'm seeing some potentially odd behavior in ERA5's volumetric_soil_moisture_level_4. In South Africa, the value hasn't changed for the past 3 years. I recognize that it is quite deep, and the response of the values to above-surface changes will be very damped, but I would expect to see some changes (unless we're in rock???).  The first attached figure shows the time series for soil moisture for the past 3 years near Pretoria. For contrast, over the central US, we do see some seasonal changes for the same time period (second figure).

So my general question is this: Is this expected behavior? If so, it there some documentation about the soil variables in ERA5 that would indicate where we can and cannot expect temporal variations in soil moisture? Thank you so much!