OK - so at least the sensor is working 
The WF team are working hard on the continuous learning rain calibration, but they aren’t there yet. As far as I know they are still using one fixed calibration for all Sky units, and this is resulting in some over-reading (i.e. they’re too sensitive) and some under-reading (i.e. they’re not sensitive enough). If the behaviour of your Sky changed after a firmware update, I imagine they adjusted the calibration and has resulted in your Sky becoming a little less sensitive.
There is lots of information on the rainfall calibration here: https://community.tempest.earth/t/rain-seem-off-auto-calibration-system-not-yet-deployed/930/88. The long and the short of it is: be patient - they will get there as quickly as possible.