Am I correct that your text field is set to Slide up and resize the enclosing part?
If so, this is a known issue.
For More Information see: Frustrating subsummary bug
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
Yes you are correct in so assuming.
I followed your More Info link, and it is the same problem, and the work-around mentioned there - selecting all objects in the report Body and all objects in the Sub-Summary Part, and annotating them all to move up and reduce the size of the enclosing part - fixes the issue.
The work-around is hardly intuitive and I really appreciate knowing about it. Thanks for taking the time to reply to my issue.
Thank you for the post.
I also added your research to the original report made to Testing and Development.
This issue has been fixed in FileMaker Pro 14.0.1.