When adding iron rune quickly it seems to not add up

Bought this chest

https://ibb.co/YhPpDsC

As you can see when you quality it, should go to 354 es, which I did.

Then I applied two iron runes, which both should add 20% cumulatively 40%.

My basic math was... 354 * 1.4 = 495.6 ES

But my item only has 428 ES

https://ibb.co/WsX9BWs

Which is a curious result. I tried 354 * 1.2 = 424 * 1.2 = 510 ES

This seems like a bug to me. I think because I dropped the runes in by spamming clicking them onto the armor, it hasn't applied the second one correctly. Please let me know if there is something I have misunderstood.

Thankyou!

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