It's obviously a caching issue, but it is very weird and long-standing (more than 10 days).
After making the tag synonym on ruSO, I am faced with the interesting behavior related to the number of questions.
If you open newest questions list by tag you will see the number 537:
The total number of pages is 11:
The same amount of questions is for the page 2. The last page (11 in this case) will give you 729 questions already:
The number of pages after that step will be increased to 15 (since the number of questions increased):
Going to the page 2 again will give you 729 questions (last time it was 537). Page 1 always says about 537 questions only, no matter have you visited the other pages or not.
Bear in mind that the visible number of questions may rely on the previously opened page.
Switching to the Active tab shows full amount of questions.
The number on tag popup contains the smallest value:
To downvoters: this issue should impact all of the SE network, but to reproduce it on other sites you should know recently synonymized tags.
Same issue on StackOverflow is reproducible here.