Skill + Flash : we need clarification !!

I recently found out that R + Flash of Blitzcrank would move the visual effect with the flash but the damage stays at the original location (seems like a bug).

More generaly I would love to have a real clarification about flash when spell is charging because there are so many different behaviors :

• sometimes visual effect AND real spell / damage are applied at the FINAL position ( Ahri's E for instance)
• sometimes visual effect AND real spell / damage are applied at the ORIGINAL position ( Morgana's Q for instance. It's a shame it's not like Ahri's E)
• sometimes visual effect are applied at the FINAL position and real spell / damage are applied at the ORIGINAL position ( Blitzcrank's R for instance)
• sometimes visual effect are applied at the ORIGINAL position and real spell / damage are applied at the FINAL position ( Cassio's R a few months ago)

WE NEED EXPLANATION !!!!

And I would love to get a real list of all the spell and their behavior with flash.