This is a Known Issue with WebDirect. For More Information see: WebDirect Bug - values for Radio and Checkbox Buttons display as row , not as list
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
Perhaps you could set up a pair of portals with buttons that look and act like check boxes for populating your text field with selected values.
See the "Check Boxes with Scroll Bars" exploration page found in "Adventures in FileMaking #2" for several different examples on how you might set that up.
Excellent. Thank you.