I have several fields that were set up as text fields since the numbers would never be used for calculation.
The Format property of a control or field needs numbers or dates. It won't recognise a string. The Format function will implicitly cast to a number which is why it works.
Bite the bullet and change the field to a Long Integer which is what it should have been in the first place. Population is a number and should be stored as a number.
(Use V_tools Total Deep Search to find places where you need to drop the string delimiter quotes in code and queries.)
The notion that numbers should be stored as text if they are not calculated is one of the most misleading suggestions ever made by Microsoft.
They meant it for fields like phone numbers and serial numbers where calculation is meaningless rather than simply not having calculations applied.
Microsoft's advice is plain wrong anyway. Unfortunately it is parrotted widely online by people who have no comprehension of the issues.
If something can be stored as an Integer (or Long Integer) then it should be stored as such regardless of whether calculations wouold be meaningless. Numbers are much more efficient than text to store and index.
I got banned for saying this at another site because it conflicted with the doctrine of their mods.