Hello
I cannot reproduce the said behaviour. Perhaps your Numbers plist file is corrupt.
You may try as follows:
1) Quit Numbers.
2) Open ~/Library/Preferences folder in Finder. In OS 10.7 or later, this folder is hidden for some reason. In order to open it, use Finder's Go > Go to Folder… menu, and specfiy the path:
~/Library/Preferences
3) Move (drag and drop) the following file to desktop:
com.apple.iWork.Numbers.plist
4) Launch Numbers and test the behaviour. Numbers.app will create new com.apple.iWork.Numbers.plist file with initial settings etc.
5a) If the said problematic behaviour is gone, the culprit was the corrupt plist file. You just need to redo some settings from Numbers > Preferences… menu and the problem will be solved.
5b) If the problematic behaivour persists, plist was not the cause and so you may restore it from desktop. To do it, first quit Numbers and then replace the newly created ~/Library/Preferences/com.apple.iWork.Numbers.plist with the one in desktop.
If this does not fix it, next I'd suspect font problem.
Good luck,
H