It's standard for Windows. Double click a .exe, and a new copy loads and runs. If it wants to quit because there's an existing instance running, the code has to arrange for that itself. (It's possible Qt does this for you by default. Depending on the app, this can simplify some things, sometimes! Maybe WinUI is the same.)

It looks like macOS Finder tries to make it difficult to run two copies of a .app simultaneously, so I imagine many programs don't check for this case, but this is Hacker News, so we can use our hacker skills to try and do what they don't want us to: find the exe inside the bundle and run it directly from Finder, or run it from the terminal using its path. (This works for me with Ptacek's mdv app. Two copies of it end up in the dock.)