msuess, I can only say that it also happened to my in a clean 3.0 install.
My workaround (not solution) was rolling back to the first version of the page, and start again.
That was very annoying, and for sure, there must be some bug around, but I haven't been able to reproduce it, unluckily.
Could you please add a bug report and set a high number for it as the importance field?
Cheers, and welcome to Tikiwiki community.