Repeat ticket starts on Sunday instead of Monday


I am trying to understand why the repeat ticket extension starts to count the week from Sunday. I have this extension installed for many years, and always it was kind of finicky to understand if the recurrence was working at the correct time. After updating to the new version, it is clearly visible when a ticket will be created. But after reading the code, I can not explain why the week on my system start on a Sunday and not on a Monday.

In my case, I want to choose the recurrence pattern “Every Weekday” so that a ticket is created on working days Mo, Tu, We, Th, Fr.

As I understand, the part that is responsible for that recurrence is the below (lines 281-287 extension version 2.02

elsif ( $content->{'repeat-details-daily'} eq 'weekday' ) {
                $set = DateTime::Event::ICal->recur(
                    dtstart => $last_due || $last_created,
                    freq => 'daily',
                    byday => [ 'mo', 'tu', 'we', 'th', 'fr' ],
                next unless $set->contains($checkday);

By reading the code, I understand that the first day of the week should be Monday. But as seen in the screenshots below, it is Sunday.

As a workaround, instead of the “Simple Mode” I use the “Concurrent Mode” with 6 days lead time.
Date, time, timezone and epoch time on the server are correct. I am out of ideas where to look.