Reproduction:
Disable auto end of turn.
Start a tutorial in version .75 or .76
Progress to the stage where you have put the workshop in the production queue.
Note the date at the top (probably spring 200 AC)
Note the number of seasons required for production (eg 9)
Click Turn
Note that it is the next season (eg summer 200 AC)
Note that the number of seasons required for production has decreased by 4 (eg now 5).
This occures with unit production too.