QtCreator keeps forgetting about Additional Preprocessor Directives

QtCreator doesn't consistently apply the Additional Preprocessor Directives (e.g. Tools -> C++ -> Additional Preprocessor Directives, or the `#` icon at the top of an editor window) when parsing a file. Sometimes the file is parsed as if the additional directives aren't present.

For example, if you have `#define FOO` in the additional preprocessor directives and your file has a `#ifdef FOO` section, sometimes that section will be greyed out and symbols defined in it won't be searchable.

One reliable way to trigger this problem is:

1. Open a file with a `#ifdef FOO` section in the editor (can be any symbol, doesn't have to be `FOO` specifically).
2. Add `#define FOO` to the Additional Preprocessor Symbols for the file, so that the ifdef'd section is not grayed out.
3. Close and reopen QtCreator, then reopen your last session.

The ifdef'd section in the file is now grayed out, even though the `#define FOO` is still there in the additional preprocessor directives.

I sometimes see this happening simply while editing in QtCreator too, not just when I restart it. The problem occurs with both qmake and cmake projects.