apns-collapse-id does nothing

I'm using libcurl on my server side PHP script to send PHP notifications via HTTP/2. In my headers I'm sending an ID, like so:

curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'apns-topic: ' . BUNDLE_ID,
    'Authorization: Bearer ' . generateAuthenticationHeader(),
    'apns-collapse-id', 'qwer1235'
]);

When the notification comes in I don't acknowledge it on the phone, and then I send another message with a different alert title. Both messages are then displayed on the phone. My understanding of the apns-collapse-id is that it should have removed the older message and replaced it with this new one.

Am I missing something?