Confirmed.
That 'master/parent send' checkbox is literally a "shortcut" for creating a send to the master. (It saves one whole mouse click as well as confuses the hell out of new users!
) Must be a clash with the normal creation of sends.
The faux pas here is this would limit you to one instance of a send to the master from any one track. Normally fine but more complex routing needs come up especially in surround sound work. You can't send from the master track either.
What always ends up happening is we use a normal (universal) track for a master track and the limited use Reaper master goes unused or is used for a solo (cuemix) bus.
My humble suggestion would be to remove that weird looking shortcut (it only saves one mouse click). Make inserting a send to the master track consistent in appearance with everything else. You could still have a preference setting to automatically include a send to the master on any new track, so that wouldn't change. Or maybe keep the checkbox but have an actual GUI element for the send appear that is again, consistent in appearance with sends.
On the other hand, a dedicated (and functionally limited) master is already obsolete in Reaper with universal tracks. So... maybe do nothing?