So I've noticed that if in a textbox if you select text from right to left the SelectionStart will be the index of your cursor when you started, this is not the behavior in WPF. In WPF SelectionStart is the index from the beginning of the text.
For example if I have the text 'textbox' and I start my selection from 'o' and drag it to 'e' the SelectionStart will be 6 instead of 1. If I select the text from 'e' to 'o' SelectionStart will be set correctly to 1.
SelectionLength is always correct though.
Is this intentional or a bug?

This is a bug, I just fixed it for the next release.
Thanks for reporting.