You might have a corrupted layout. Does this happen to several layouts or only one? Have you runa. recover on your file lately? If you do, do nto replace your file, just check what's wrong in it (gets an error) and correct those errors manually, if you can.
What happens if you create a new layout and work on that, do you see the same problem?
You have to narrow this down, not just keep repeating the same process and hope for a different result.
Hope this helps,
It could also be that you have been saving changes to the Styles but not to the underlying Theme. Styles saved but without the Theme also saving those object changes, only affect the current layout.
When you save changes at the Theme level, note the Theme name, and whether or not it changes, then go thru the layouts as bee sure the saved Theme is actually chosen for each where it applies, rather than some unchanged version of the theme.
This can also be tricky because FMP doesn't let you change the built-in themes except at the File level. So, if you do change a theme in a multi-file system, you also need to import that theme into the other files and then go thru the layouts to select the imported version of the Theme.
Lots of levels at which to get tripped up with Customized Themes, especially in a multi-file solution.
This happens to me too. It's so annoying!! I have all styles and themes "saved" and "up to date" and sometimes I'll go to edit one field and the entire layout loses all styling. It's very frustrating. I have to then apply styling to each field. It happens to me on my Mac version of FM and also to a coworker on this PC version. I really think it's a bug.