Surplus is not estimated (although used) in shared project. This may cause confusion.
As the save and pic implies, city has 107 surplus + 323 industry > 350 for Pyramid, but on estimation surplus is not shown, so 2 turns instead of 1 is shown as estimation.
Step by step of the bug
Step 1
Start a shared project (in this case Pyramid of Giza) when surplus industry >0
Step 2
Check estimated progress, and surplus is not calculated
Surplus is not estimated (although used) in shared project. This may cause confusion.
As the save and pic implies, city has 107 surplus + 323 industry > 350 for Pyramid, but on estimation surplus is not shown, so 2 turns instead of 1 is shown as estimation.