I have been noticing this bug lately and I think I understand what is causing the problem. It seems, when I approve an empire plan with Building Cost Reduction and/or Unit Cost Reduction, while I also have the corresponding Building and/or Unit to be completed in Queue, the Queue will get stuck with that item claiming 1 turn left, 100% production meter progress, and will not complete advance to the next item in Queue. This is highly aggravating.

I suspect the problem lies within the equation, or order of equations, to resolve the Cost Reduction between turns, but only during the resolution between turn 20 and 21, or 40 and 41, etc. I was wondering what would happen if the % reduction were applied to a negative production value? For example ... having 50 production with 30 remaining to complete ... [(30-50)*0.66]. Whatever it may be, something is causing the resolution of the production to fail.