Falconer's Jacket not increasing movement speed

The tooltip on Falconer's Jacket says "5% increased movement speed". Yet it increases my movement speed by only 0.3% (from 51% to 51.3%). That's an increase of 0.5%, so it's not a 5% multi nor a 5% stacking (which I'd expect should be the case).

There is a known hidden movement speed penalty on types of body armour used. I do not know exact numbers but the heavier the armour bigger the penalty. I think it goes up to 5% movement speed penalty.

That's not the case here. This armour has an implicit "+5% increased movement speed" and it's not a heavy armour, according to poe2.db this type has a -0.03% reduction in movement speed, so it's negligible.

The calculation/tooltip of Falconer's Jacket are simply wrong, no matter how you look at it - whether from a +5% multi perspective (if that was the case I should have 53,55 movement speed) or a +5% additive (like boots and passive skills, which should make my movement 56%). The calculations are off or it has hidden, very high diminishing returns, however that would make it a terrible end-game armour, and it's supposed to be a chase item.