I tried your use case. I have this .htaccess at the root of my site:
RewriteRule ^properties/([0-9]+) properties/showprop.php?id=$1 [QSA]
And I created the page "/properties/showprop.php"
In "showprop.php", I have this code:
echo "id: " . $_GET["id"] . "<br>";
echo "UTM: " . $_GET["UTM"];
When I call the URL http://localhost/properties/11?UTM=10
The page prints out the expected result:
Show prop: id: 11
So, either I didn't understand what was your question or there's something else preventing it from working.