That is definitely a weird bug... I can't reproduce on any of my installs. Do you have any other plugins active that utilize the WordPress taxonomy system? Or maybe does your theme does things with the WordPress taxonomy system?
It sounds like a theme/plugin conflict happening here. At any rate, in your specific case I consider this resolved b/c you have a workaround. If you want to do more investigation to try and figure out the cause I would suggest doing the following (all the steps below are just temporary to troubleshoot):
1. Switch to the default twenty-eleven theme that ships with WordPress. Are you able to reproduce? If yes, then you know it's not a theme issue. If no, then you know it's your theme causing the confict - no further steps necessary - just let me know the theme.
2. Deactivate all your plugins except for Organize Series. Are you able to reproduce the issue? If no, then one by one activate the other plugins and try to reproduce the issue. Once you discover the plugin that causes the conflict let me know.
If you do investigate this further and find out if it's either a theme or plugin conflict let me know and I'll take a gander if I get a chance to see if this is something needing fixed on my end.