Hi Sean,
I've not seen this issue or had any experience with it, so these are a pair of general trouble shooting suggestions used to clean some of the crud accumulating over time.
- Press and hold the shift key while you relaunch Numbers. Keep shift pressed until Numbers has completed the launch process, then try opening a new document from a template.
2 Similar actions, but this time a startup on OS X into Safe Mode.
In the Apple menu, choose restart.
Press and hold Shift as soon as you hear the startup chime.
When the Apple logo appears on the screen with a progress bar shown below it, release the shift key and wait for the reboot to complete.
The Mac runs several more cleanup routines in this type of start, and you will see several behaviours on screen that do not appear during a regular startup. You'll also see a distinct lowering of visual presentation while in Safe mode.
Once the restart has finished, go again to the Apple menu and choose Restart , and let the Mac shut down and restart without interruption.
Regards,
Barry
PS: " I'm tempted to try deleting the app and re-installing it, but I'm not sure if I can get the old version from the App store… "
If you have Time Machine, or another backup system running on your Mac, you may have created a 'restoration' section from which you can restore a working copy of your current Numbers application.
The link below is to a post discussing updating the names application to a newer, but not current, version. The steps described might also work to 'update' to the same version as you are running. but like you, I would hesitate on deleting before going for the reinstallation.
Best (and Apple Recommended) answer regarding installing more recent versions is found in this post by Viking OS X in the Pages for Mac community: pages, numbers and keynote
You might also try contacting Apple Support directly regarding reloading Numbers.
B.
