Multi-carrier itineraries are notoriously difficult to get correct. The rules that apply are Thai’s, but it’s Brussels that has to enforce them, and it’s United that’s supposed to code the appropriate value into the ticket since they’re the ones selling it. There is a non-zero chance that they attempt to apply some other policy. My advice is (a) to bring your receipt; (b) be prepared to pay something for your luggage, even if it shouldn’t be necessary; and (c) be prepared to file a refund request against UA, even though they’re not the ones who’ll charge you, because the DOT requires that they issue accurate receipts and it also requires that they give you accurate information throughout the process. You should be able to rely upon the 2x23kg booking confirmation, even though it’s entirely incorrect. (I assume you also get the same 2x23 kg value listed during the booking process?).

It looks like the allowance for Brussels Airlines is 1x23kg. The LH group appears to use a per-piece system similar to what US airlines use.