No, did not change from Canada. As I said, I have no explanation for why it went from 24H to AM/PM when I tried right after you said there was a bug. Then when I saw Pauli say it worked, I tried it, again and yes indeed it held.
Why it seemed 'broken' to you & then was not behaving badly, I won't hypothesise and have you suggest I am being "silly", again.
It was said "Show abbreviated" did the same and that too was changed to not being the case. seems all one can say is "all's well that ends well".
And apparently, also, contrary to what was said initially, 24H IS available in the US. Often the US is the (global) exception, but not offering 24H time in a global OS was very surprising.
Retail 11 Canada without seconds
Same with seconds
Cheers,
Drew
 or English (Canada) while they work fine if I select English (Europe).
What is weird is that you saw the same and then the problem disappeared. 😂😵💫🤷♂️
Also, you screenshot was taken in the morning, these below after 12 noon and before midnight. 🤷♂️

Here is what it shows in the morning, set for 24Hours format:

adds the AM when Show Seconds is enabled. 😂😂😂😵💫🤷♂️
