I haven't seen this issue reported anywhere with the last release of Keynote, running in High Sierra or Mojave, none of our systems our our clients have this problem.
You have not provided the versions of both Keynote and Mac OS that are being used, so let us know what these are.
Try resetting Keynote as follows:
- ensure you have a current backup of all your user files saved to a separate drive
- restart the Mac with the shift key on the keyboard pressed down, release shift when the Apple icon appears (Apple Menu > Restart). Start up will take longer as the Mac is performing a software repair or the startup drive.
- ensure all applications are closed
- uninstall Keynote; for this procedure to work, an application removal tool must be used, App Delete is free and works for this procedure. It can be downloaded from the developer’s website here: App Delete
- empty the Trash
- restart the Mac normally: Apple Menu > Restart
- reinstall Keynote from the Mac App Store then test Keynote