I would add that these issues seem very similar to the ones I experienced with my Sharky 775 heat meter as detailed in this thread.
https://community.openenergymonitor.org/t/sharky-775-heat-meter-questions/
Not sure what help that is, but just corroborates the positive numbers of the Sontex.
When I swapped out the Sharky and replaced it with a Sontex, all my flow rate recording issues went away overnight.