Possible calculation error in Thornmail damage.

It seems that the damage on thornmail isn't calculating properly after numerous tests with its theoretical damage including it being reduced by magic resist the item isn't performing as it should be. I am aware that only basic attacks and things like Ezreal and Yasuo's Q cause its effect. However the current error seems to be several which I am still trying to capture as a picture.

Possible Thornmail Error List:

Bonus damage from Sheen / Triforce not being counted towards return damage. Critical hits are being calculated as regular hits when returning damage. Before damage reduction is not properly calculated.

My test for the final claim will be test an itemization that should return more damage than the user deals. Given that 300 armor is roughly 75% reduction in a 100 dmg attack the Thornmail wearer should take 25 damage and the attacker should take 30 (if they have no MR).

I'll post more as I get more evidence on what is an error and what is simply not tool-tipped correctly.