Never noticed it before 3.5… But at least 90% of the time when I collect tributes. The amount of gems I get is always 1 less than the total amount of Kingdoms I’ve received tribute from. A little to spot on to be true RNG in my opinion. Seems like a gem cap that keeps me from having too many or too few gems.
(I still think tributes were nerfed to off set the gem increase in production from daily tasks).