This is not a huge issue since NB stays running and I don't reboot more than once a week, unless absolutely necessary.
I added the MemCacheLimit and increased it to 55MB in the "settings" section of my default .nbi file.
I then added a few new groups and saved my .nbi file. The MemCacheLimit entry was removed and NB reverted to the default 20MB setting.
This also happens when I close NB and restart it.
In order to get the 55MB setting, I must edit the .nbi file and then start NB.