When you close a project, then reopen and make changes to the FrameInfo of a frame in Elements Explorer, then close the project again, those changes will be discarded.

Because element definitions (and their frames) are actually contained in test files, doing anything to the test itself which uses that element will mark the test "dirty" (it will be in bold in the Project view), then the changes will be saved. Simply unchecking and rechecking the step enabled checkbox is sufficient. The trick is that you must do this on the test in which the element is actually used by. Doing this on just any random test will not be enough.