I'm in a similar situation here.
I have one Tvheadend server I set up in mid 2021, that uses a "legacy configuration dir" according to APT logs whenever Tvheadend gets updated, and that displays the date correctly in British format (day before month).
However, I set up a second Tvheadend server at a different location in mid 2024 (specifically July 2024), and that uses the new style configuration format, but while the time is correctly formatted in 24 hour time like the other one is, the dates display incorrectly (for me) in American format (month before day) despite me setting the UI font to "English (GB)".
I also noticed some text strings being different despite both servers having the same Web interface UI language option selected, like in programme info where there's a button labelled "Autorec" on the 2021 server but "Record series" on the 2024 server.
For both, I suspect they're bugs that only surface on Tvheadend installations installed after the new configuration folder location was introduced.
Both servers run Tvheadend on Debian 12 (though on x86 instead of ARM) and are running the latest build as of posting this, 4.3.2366.
I had to do a Web search to find the format codes Tvheadend expects for custom date and time formats, which not only led me here, but also to this page that does list the format codea.
I figured out the date format to use, which fixes the date format in lists on the guide and DVR tabs, but messes it up in the programme info dialogue, however it's the best I can currently get it until this bug gets fixed.
%dd/%MM/%yyyy, %hh:%mm:%ss
If I have access to a desktop Web browser within a few days of posting this, I'll update this reply with screenshots showing the Tvheadend admin panels on both servers to show the differences in date formats and similarities in UI language settings.