Unless you are playing on two separate accounts (which is a no no) the diamond counts would always be the same on all of your worlds. If you think that you are short diamonds, you need to submit a ticket to Support. They will be able to figure out what may have happened.
As far as I'm aware Diamonds have always been shared between worlds. The only intended exception would be the 100 diamonds during the Tutorial.
"oh wow, I am about to be notified that I won some diamonds!"
Not entirely true. You can have two accounts, you just can't have more than one city per world. Technically you could have a different account for each world, although that would be rather foolish.
This is the real oddity here , 9 years ? and you didn't know that diamonds are shared across worlds?
I think I've seen this same paradigm (if I'm using that word correctly in this context) on the awarding of medals. There is a slight pause and then the medals are placed in the inventory. Strange, I first noticed this when working on gb leveling parties and you can see the medals' deposit just before the gb actually shows that it's leveled up. Guess that made me aware of the slight "hiccup."As Inno once explained (revealed), when a bug hit the game that affected diamonds. They are of course carefully monitored, and are from their own server/service.
In fact, students of the game can accurately and repetitively guess correctly about your GbG reward earned when diamonds are won because... there is a palpable but very short "blip" (tiny pause) when the reward is revealed to you for the advancement you just made that probably comes from the extra accounting/servicing of true game currency (diamonds) vs things like coins or supplies. You can get pretty good and correct before the award message box is revealed. - with practice, of course, and over time you play GbG.
In GE too, same paradigm. Slight pause when you win diamond rewards for encounters.
It can be confusing sometimes especially with the pooled diamond fund. Basic rule of thumb on issues involving your currency, contact support if you can't find what you want to know if the FAQs. I've always found them to be helpful.