Excellent find and totally reproducible ... thank you for reporting this - I had not seen this. 👍🏻

Now comes the not so great part; RMTV takes exactly what the TVDB API provides.
So these "season" and episode number are 100% copy of what the API provides.
Note: oddly enough, API and website are not always in sync. Like they use different databases (which would make sense, considering how the database are used).

Now ... Maybe the new API (v4) does not have that problem, but there are a few reasons why I haven't implemented this;
- this will take quite a bit of work on my end,
- all users will have to get a subscription API, 
- I, as the developer, will have to get a subscription as well,
- TVDB's track record hasn't been the greatest in the last few years since they keeps changing things (this would be the 4th time now for me), unreliable, inconsistent, etc.

I wish I could show this with the old testing environment, but TVDB seems to have removed the v3 test environment in favor of v4. I just tried going there, couldn't find it with Google either. And ... I cannot even test v4 unless I pay a subscription fee.

The subscription is not insanely expensive ($12/yr), but these days we all have to watch how we spend money, I will not get the subscription until I'm 100% convinced I want to implement v4 for RMTV.

I'll pin this one on my desktop - maybe even dig in a little deeper this week.
Again: thanks for reporting!

Maybe that's interesting for you: Plex shows the episodes in DVD order correct. But idk how they do it.

Anyway, thanks for the quick response! I'm looking forward to a fixed version!

That most certainly is interesting. My best guess: they use API v4.
After all ... TVDB has been pushing for 3 years now for all to go to v4. v3 doesn't make TVDB any money, where as v4 will make them potentially a ton of money.
And I can only guess, since the test environment for v3 is gone, and the test environment for v4 requires a subscription.

TVDB offers two ways of "paying":
- I, the developer, negotiate a bulk license (we're talking a lot money - buying a house will probably be cheaper)
- You, then end user, pay $12/yr

So, if you, as a user, are not paying specifically for TVDB (fill in user-account and TVDB pin code), then I'd guess that Plex will have worked out a commercial contract with TVDB. Which would not surprise me, since most Plex users have a paid subscription for Plex, making it that Plex has the funding for that. I do recall Plex and TVDB arguing about this though, and I think KODI even dropped TVDB support because of it.
Anyhoo, to put that in perspective; I received a total $150 in donations, over a period of 4 years, for RMTV.
I'm not doing it for the money, obviously, just saying that a bulk contract would not be an option for RMTV.
Maybe I should go more commercial as well? 😉  (I would not like that)