Custom Profile Textbox Field Strips Characters in Default Value

Characters in a json string are stripped on save after entering the string as the Default Value parameter for a Textbox field. This string:

[{"membership":[{"name":"Silver","background":"B3B3B3","label":"141619"},{"name":"Gold","background":"BA9C00","label":"383000"},{"name":"Platinum","background":"9C9D9E","label":"141619"}]}]

becomes this after save:

[{