I don't see you doing anything that could be confused with stiff leg, until you got to your 405 set. A couple things that stood out to me:
1) I really don't see you setting your back at all. This is especially noticeable on the sets filmed from the rear.
2) you may want to try turning out your toes and shoving your knees out. That will get your butt a little lower which will make it a little easier to set your back. also your knees were kinda caving in a little, shoving them out may help with that.
Other than that congrats on the PR! And without a belt! Have you tried DL with a belt?