The sensed pressure steadily decays to ~zero after about one minute. I first observed this with the Sensel App in Visualizer mode. The pressure signature of a placed object slowly decays until it disappears (object not moved, no overlay).
I tried again using the SenselForces example project which tabulates the total force for each frame sample. I placed my iPhone face down with no overlay and recorded the total force. The decay is shown below.
Is there an API setting to turn this off? Hopefully it's (just) a software issue.