Could this be a nother aspect of this bug just acknowledted by FileMaker today?
For More Information see: Can't select hairline boxes in FM 12.0v3
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
Thank you for your post.
This was previously reported here:
I added your research and the sample file I created that replicates this behavior to the original report.