It is very annoying, but this only happens with a button on a layout converted from an earlier version of FileMaker.
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 Phil. It was indeed a layout from a converted file.