Have noticed over the last few weeks a drop in all mine production on just one of my planets... Made a point of checking values of Metal , crystal and Duet.. left for 12hrs to get an accurate reading.. Production was just a few thousand instead of the millions it should have been.. Yes production is set at 100% on all mines and energy is more than enough.. Yet very little increase from when I left it 12hrs earlier. This must be a bug there is no other explanation.

Sounds like you are missing out something. Game bugs do not happen like this.

Take a record of your current resources and store them somewhere, before comparing again them at a future date. Present any differences found here and I will check then myself.