Yet another wrinkle: I start by opening a different presentation,
Presentation Y. I close Y and File > New > Blank presentation and it
reopens Presentation X.
Did you close X before opening Y? If not, X wasn't re-opened, it was just
revealed again when Y closed.
The business about it not doing anything when you choose to create a new
presentation ... well actually, if you have the task pane closed, it'll reopen
on New Presentation when you do this, but if it's already open ... nothing
happens. You need to click one of the options under "New" in the task pane.
No, don't tell me. I already agree. DUHHHHMB interface design. ;-)
Now why it should keep opening with the last-used presentation already open ...
no idea. Is PPT crashing when you quit? If so, it'll try to start up again
with the most recent presentation recovered and open.