Having just encountered that issue myself. I was putting an apostrophe (') in the name of the manual test. Support told me that it is a forbidden character, however it is undocumented as the help does mention forbidden characters. The ones that are listed give you an error. They have already logged this as an enhancement.
The easiest way around this is to take the special characters out of the name, and save. DON'T refresh the script info center, right click the manual test in the list, and select 'Duplicate'. Delete the original, and remove the  from the end of the new test. You will notice, if you rename the test that is having the problem, the steps will return. You can refresh the script info center, and the steps will stay. But when you add it to the library, and open it from there, the steps are gone again. It is buggy. Do the duplicate thing, that will keep you from having to retype the steps in the test.