[Update somewhat later in the day]
It wasn't the BlogEngine.NET people's fault. With a little reflection (the wetware, not software, kind) I realized that was probably the case, and went and checked around in the settings.
The way the page links are being generated now is optional.
Sigh. If I'd realized that before I went and changed all my internal links, I probably would just have changed the setting.
Not changing them back now, because this is better functionality. It allows a page title to be used more than once. Besides I couldn't face going back in and re-fixing my internal links.
I can't even blame the BlogEngine.NET folks for setting this feature to be the new default, not even from a backward-compatibility standpoint, if I want to be fair. This change occurred in v 1.2, the current version is 1.4.5, and I'm probably the only one who didn't already know about it.
And let that be a lesson to me.