Diamond use confirmation is active in my settings.
However: in the Aztecs settlement I did not get the warning box when (accidentally) clicking on the box to reduce time for goods production.
This has cost me 100 diamonds:
first time was accidental, second time on purpose to check if indeed diamonds were deducted - and yes they were deducted
Anyways: not a nice bug for unsuspecting players who click on the time reduction box accidentally, so BEWARE!
P.S. YES, I have contacted support about this bug