The "Edit own thread title" permission is really "Edit own thread" as it allows editing a bunch of non-title properties. This appears to be a holdover from XF1.x
Dropping the "title" from the permission text probably makes a lot more sense
We've changed this now but note that the permission ID is still editOwnThreadTitle as changing permission IDs can be a little bit dicey given that there could be add-ons who rely on that permission currently to be named in that way.