This was driving me nuts, and I finally decided to search for where a page template’s value is stored in the database. This gave me my first clue, because I found single-sponsor.php, the name of one of the other template files I’m building.
I was confused as to why this could be the case, and looked at the HTML of the actual dropdown menu WordPress generated:
<select name="page_template" id="page_template">
<option value="default">Default Template</option>
<option value="page-custom.php">Custom Page Example</option>
<option value="single-sponsor.php" selected="selected">Market Main Page</option>
</select>
Pretty clearly choosing single-sponsor.php.
Why would it be doing that?
Because I told it to.
I had copied the code from page-market.php into single-sponsor.php to start developing a new page template.
At the top of single-sponsor.php?
<?php
/*
Template Name: Market Main Page
*/
?>
Right.
Stupid mistake. But I thought it was worth writing up, in case the same thing happens to someone else.
I changed the name before realizing that I didn’t want this file to show up in my template list, so I removed the code.
Foo.