Thank you for your post.
I have not encountered this issue, and to date, this issue has not been previously reported.
Does this issue of losing styles occur on a specific layout or several layouts? If a specific layout, what theme is being used? Try creating a new layout, apply a theme, add some fields, apply some styles, etc. I understand this issue is not easily reproduced, but occasionally, return this layout and do some editing. If this issue is occurring with several layouts, then create a new database file, apply a theme, add some fields, apply some styles, etc. When the issue occurs again on your original file, immediately open the new file and see if the same issue occurs. This will let me know if it is file related or FileMaker/system related.
Since this is also happening to a coworker under Windows, what can you tell me about the coworker's database file? That is, what theme is being used, the objects on the layout, etc.
Any other information you can provide may be helpful in narrowing down the possible cause(s).