What version of Filemaker 12 are you using? The 12.03 updater was supposed to fix this issue.
For More Information see: After making a line a button, it will delete the line when going to browse mode.
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
If you are using FileMaker 12.03 or the latest update: 12.04, let us know. As a work around, you can try using a rectangle object sized to 4 pixels wide and filled with the desired color.
The version of Filemaker Pro is 12.0v1 and is on a Mac using OS X 10.8.3
You need to download the free updater. Latest is 12.0v4. One of those updates fixed this issue.
Philmodjunk, The update to 12.0 v4 fixed the problem. Thank you very much!