I’m including the entirety of my test page. I was initially having this problem with Zoo Visitor, but I pulled it out to test the underlying capability of Channel Form, and discovered that I have the same problem here.
In EE 2.8.1, this code properly displays the options for the related field. But when the form is submitted, the field gets cleared. If I update the related entry in the back end, this front-end form will display the correct item in the dropdown until the form is submitted, and then it wipes the relationship.
Any ideas? Am I doing something wrong? I got this code directly from the documentation.
{embed="main/header"}
{exp:channel:form channel="zoo_visitor" return="/main/success" entry_id="1"}
<select name="expertise">
{options:expertise}
<option value="{option_value}"{selected}>{option_name}</option>
{/options:expertise}
</select>
<input type="submit" name="submit" value="Submit">
{/exp:channel:form}
{embed="main/footer"}