The Thalys ticketing system is flawed

That "i" icon pop-up would be evidence enough for us. You can save or print a screen shot if you feel inclined. While this is new to you, it's not to the railway conductors and they will all know with the rule. Realistically, they can't even check tickets during the course of that 3-4 minute train ride.

You are right in that they don't seem to have this language anywhere on their website, although many unofficial websites do confirm that a Thalys ticket is good for making a transfer within Brussels on a domestic train -- see seat61, a TripAdvisor link

They didn't sell you a ticket to Brussels Central because Thalys trains don't call at Brussels Central (the only Brussels station they stop at is Brussels Midi), although you will physically pass through there when coming from Amsterdam as Brussels Midi is the southernmost station on the main north-south railway line through Brussels.

It is worth noting that the journey between Brussels Midi and Central is 3 minutes via train, that a standard ticket would cost €2.40, and that during the day there are trains every couple of minutes, because all domestic Belgian trains stop at the three main Brussels stations: North, Central and Midi. Given the very short travel time I agree with Laura that it is vanishingly unlikely that you will encounter a train conductor between the two stations, but should you really want to reassure yourself you could always purchase a single €2.40 ticket as insurance.

I think you will find that boarding one of the frequent trains from Midi to Central is rather like getting on a subway and getting off at the next stop. If, by some rare chance a ticket inspector asks to see your ticket, show him/her your Thalys ticket.

Because they don't need to. Any ticket from anywhere to Brussels is valid to all 3 Brussels stations.
It's Belgian Railways standard ticket rules, somewhere on their website. Similar rules apply in some other cities with multiple stations in the centre.

Andrew, I did read on that info on the seat61 website, and that is great, but it doesn't represent Thalys.

Chris F. said:
"Because they don't need to. Any ticket from anywhere to Brussels is valid to all 3 Brussels stations.
It's Belgian Railways standard ticket rules, somewhere on their website. Similar rules apply in some other cities with multiple stations in the centre."

I disagree. They do need to state this clearly on their website. This info is not on their website.
It isn't really a matter of a two dollar (or euro) fare between Midi and Brussels. It's more the principle of it. There should be no confusion or doubt about what was purchased or if it will work. I should never have had to ask all of those questions to the Thalys customer service representative.

"Just for the record, I never said it represented Thalys and did note both that it was "unofficial" and agreed with you that you couldn't find this anywhere on the Thalys website."

Sorry Andrew - I didn't intend to come off as argumentative. You are correct. I just meant that while the info was very helpful to me in that it broadened my understanding of how Thalys worked, because it didn't actually come from Thalys I couldn't rely on it to get me out of a jam if someone wanted to see my ticket from Midi to Central.

I realize that many readers may be thinking "big deal, it's three minutes away". I just feel that Thalys should be obligated to issue a ticket with words that give the final destination on the ticket, rather than the transfer location.

You are overthinking this, like others have said it is standard SNCB/NMBS ticket rules. Every day, hundreds if not thousands of passengers arriving at Gare du Midi on Thalys or Eurostar do it. The staff know the rule. If you want something official, the information is available on their website: https://www.belgiantrain.be/en/tickets-and-railcards/standard-ticket (only in French and Dutch though)

And in case you are wondering, the Brussels zone is made up of the following stations:
Arcades, Berchem-Ste-Agathe, Bockstael, Boitsfort, Boondael, Bordet, Brussels-Central, Brussels-Chapelle, Brussels-Congress, Brussels-Luxembourg, Brussels-Midi, Brussels-Nord, Brussels-Ouest, Brussels-Schuman, Delta, Etterbeek, Evere, Forest-Est, Forest-Midi, Germoir, Haren, Haren-Sud, Jette, Meiser, Mérode, Moensberg, Schaerbeek, Simonis, St-Job, Tour et Taxis, Uccle-Calevoet, Uccle-Stalle, Vivier d’Oie, Watermael

It is a little tricky. I had one of those tickets Brussels Centrale to Paris (change in Brussels Midi) and was thinking "is this legal. are they gonna give me fine if they check my ticket." But at least you have 4 months to sort it out.