Open Type properties glitch?

Hi,

I have an issue with a block of type that is using OpenType stylistic set 3 (so it has a round 'a' rathar than one with the tail at the top). When I copy it over to any other document it then loses that stylistic set and the 'a's all change to the kind with the tail at the top, also slightly altering the layout of the paragraph.

I am working on a series of books which all feature this same paragraph so had originally copy and pasted it over until I realised it was changing the OpenType setting.

I then tried stripping back the document which features the working paragraph to just that paragraph (with the working 'a's) add all the copy, images, etc onto this 'clean' templated document and checking the 'a's after every paste. The paragraph stayed the same and 'a's remained as stylistic set 3. However, when I then closed and reopen the document that stylistic set is not applied, the 'a's have all turned to the ones with the little tail at the top and, when clicking on the 'O' at the bottom of the text box is says, "OpenType properties are not applicable"

I've tried this twice in case I had made a mistake, but it is changing once it reopens. What can I do? Is there a setting or is the file somehow corrupt, and if so please does anyone have any solution for fixing it?