This is a known issue, but not one that FileMaker Inc. identifies as a bug as they have documented it in a knowledgebase article.
IMO, the programmers who created the file conversion code didn't do the job like they should have. This thread refers to buttons instead of layout text objects, but I suspect that it's the same issue.
For More Information see: Buttons spontaneously change size and position when a text style is applied
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks for the followup. I consider this one of the biggest PAIN IN THE A$$ bugs of FM12, in no uncertain terms. Just because there is a KB article doesn't mean the behavior is appropriate or desired. What do you suggest for getting this into an official channel if this isn't already - as I'm prepared to go to the mattresses on this useless behavior.
In addition to reporting it here, you can use this link: http://www.filemaker.com/company/contact/feature_request.html
And there is also the Feedback section of this forum. You can click the tab for that at the top of this screen.
Thank you for posting.
Development and Testing was aware of this issue. I have added your post to the original report.