Support

This makes no sense. The code which sends the email is triggered by a built-in event hook in the ticket object's save handler. It doesn't matter how you save a new ticket – web, email, or creating it with a CLI script.

Is it possible that the difference is that tickets received over email are handled by a CLI script which runs under a different PHP version (or at least a different PHP configuration) which might prevent emails from being sent?

I am afraid this is definitely not an issue with ATS 5.4.2 and / or Joomla 5.4.1. I just tested that exact configuration and I get the emails just fine when I send a ticket through email AND when I send a ticket through the web. This was completely expected. As I explained, the mail sending code is actually triggered by the Table object. It does not care where it's called from. That's the whole point of using the Model-View-Controller architecture in Joomla: there's a single point in the code which handles things instead of copying code all over the place and struggle to keep it up-to-date with each other.

I can also tell you it's not an issue with your mail templates. If it was missing, you'd not be getting any email regardless of how you send the ticket. Likewise, it's not a problem with your ATS configuration.

This brings us back to how you send emails and how you run mailfetch. If you are not using SMTP (e.g. using a third party plugin) it could be failing under CLI but work on the web. If you are running mailfetch under CLI there might be a configuration difference, OR there might be a third party plugin throwing an error when you try to send an email from CLI; do remember that Joomla 5 and later versions do have two plugin events which are triggered when you send an email through Joomla.

If you are using wget your issue makes a lot less sense than it made originally, and I have to note that it did not make any sense to begin with. I would also like to note that did try to reproduce your issue yesterday, but I was receiving the email just fine no matter how the ticket was submitted, and whether the mail fetch was running from CLI, or a Joomla Scheduled Task (therefore the web, the same as using wget for all practical intents and purposes since it's the Joomla application that matters in this case, not what loaded it).

At this point, I am thinking that your issue description may have been unintentionally misleading, e.g. because the people you asked had in their mind emails they received about tickets before the update was installed. So, I am going to disregard that part of your issue description and treat it as "suddenly" not receiving any email for submitted tickets.

I am also noting that based on your past tickets you did have 5.4.1 installed and working fine, so I am focusing only on the changes between 5.4.1 and 5.4.2. I am also noting that Joomla 5.4.1 introduced no changes which could affect anything, let alone email sending. The x.4 releases of every major Joomla release starting with 4.4 are extremely conservative, only addressing security issues and major bugs. They are the LTS version. Breakage can and does happen in the x.0 (guaranteed breakage), x.1, x.2, and to a lesser extent x.3 versions.

With that in mind, the only thing I can possibly think of is this change in 5.4.2:

Notify Support Staff option in ticket category is effectively ignored

Is it possible that you had misconfigured who gets emailed about new tickets, but because this setting was ignored everyone was receiving email just fine?

Do note that this is incongruent with your original issue description where you stated that tickets submitted over the web do result in emails being sent.

As for troubleshooting email sending, I have bad news for you. Since we just use Joomla's mail templates there's nothing for us to log. Joomla itself doesn't keep any email logs either. Beyond looking at your mail server logs for sent emails and your web server's error logs for any PHP errors there's nothing else you can possibly look at.