I've also used 4.5.1 and noticed this behavior. Upgraded to 5.0.0, but it didn't help, bug still exists. I'm not really sure is it connected with number of properties or anything like that. I created sample test case, added 30 properties and was unable to reproduce this bug, however in I my working project it happens quite often and always when number of properties is 20+.
The only way to fix it is to export properties to file, fix properties in text editor, completely delete Properties Step, create new step and import properties from file.
OS Version: Windows 7
