Boots of Striding and Springing doesn't work properly

The texts says it makes the character 'Unaffected by heavy armor penalties'. But the game doesn't add the dex bonus to the AC.

The other parts work. 

Might of the Iron Legion Feat wasted. :(

This means your movement speed will not be reduced by Heavy Armor as it normally is.  The movement penalty is not mentioned anywhere, including on the wikis [EDIT: that is because there are none in Solasta. My mistake: Heavy Armors in Solasta do not reduce movement speed as such].  The magical boots allow the character to move at minimum 6 hexes even if Heavy Armors usually give a movement penalty.

• Heavy Armors do NOT add any Dex bonuses to AC. The description may be unclear for the boots, but they are not supposed to change the lack of Dexterity bonuses for Heavy Armors.
• Medium Armors allow +2 Dex bonus maximum to AC.
• Light Armors consider the entire Dex bonuses.

Might of the Iron Legion feats adds the Heavy Armor proficiency to your character if you do not have it already (and +1 Str, and Longswords, Greatswords, Battleaxes proficiencies).  Heavy Armor is good mostly for characters will more Strength than Dexterity.  Some Heavy Armors give more AC than the combination of Medium Armor and +2 Dex.

Heavy armors preventing to use dex bonus on AC is a penalty. The description of the boots states it removes penalties provided by heavy armor. It doesn't. The text doesn't specify movement penalty, but generally penalty. It might be a text bug, but still a bug, and it's quite annoying.

The description could be re-written : not long to do. 

I honestly never interpreted the description that way, but I can see how it can be.

EDIT: it turns out Heavy Armors in Solasta do not reduce movement speed.