Champion Hoplite upgrade giving wrong amount of pierce armor

Hoplite's base pierce armor is 25%, reducing 20% of incoming damage. Multiple sources of armor stack multiplicatively, so 25% base and 50% from the Hoplite Champion upgrade should result in 87.5% of pierce armor, reducing 46.66% of incoming damage. Instead, after researching the upgrade it results in 56.25% pierce armor, reducing 36% of incoming damage, just like if the upgrade is giving 25% pierce armor instead of the expected 50%.

 

Can you please double and triple check everything or give an example of how it results in 36% of damage being mitigated as shown in the screenshot? I'm getting a bit confused here.
With base armor I think you meant that 0.20 number from there, right? And with armor value from upgrade you mean the 50% extra pierce armor.
So it should be
Armor = 1 - (1 - 0.2) / (1 + 0.5) = 0.4666... which should appear as 0.47 in game. But in that screenshot it's 0.36, just like if the extra armor is equal to 25% when the tooltip says 50%.
Please correct me if I did a mistake somewhere, I couldn't find any info on how armor works anywhere and I figured it out by myself, maybe I got something wrong.