Submissions not being delivered - don't know where to start with reporting problems accurately

I've had this working before on other projects, but not this time.

If I remove the line <input id="beeUnit" name="beeUnit" type="text" class="u-visually-hidden"> from my form it works fine (lots of spam though!).

I have the following in my contact-form-honeypot.php file:

return [
    'honeypotParam' => 'beeUnit'
];

On submission, the form transitions to the 'thanks' page, but I don't see anything delivered.

I am using the MailGun plugin/service to handle my emails.

I have the same problem on my dev (MacOS, Vagrant) and my production (Ubuntu, LEMP) environments.

Don't know where to start debugging this. web.log looks fine to me. Plugins are installed and working fine (i.e. I can visit their settings pages in the Dashboard).

Additional info

Craft version: 3.7.37 Pro
PHP version: 7.4.27
Database driver & version: MySQL 8.0.27
Plugins & versions: Latest available of both Contact Form (2.3.0) + Honeypot (1.0.2)

craftcms / contact-form-honeypot

Submissions not being delivered - don't know where to start with reporting problems accurately #10

Additional info