Dwarves have a special trait, see PHB page 20:
Speed. Your base walking speed is 25 feet. Your speed is not reduced by wearing heavy armor.
It refers to the Chapter 5, see more information here - Does Heavy Armor reduce your speed?
If the Armor table shows “Str 13” or “Str 15” in the Strength column for an armor type, the armor reduces the wearer’s speed by 10 feet unless the wearer has a Strength score equal to or higher than the listed score.
However, under "Variant: Encumbrance", the PHB says:
The rules for lifting and carrying are intentionally simple. Here is a variant if you are looking for more detailed rules for determining how a character is hindered by the weight of equipment. When you use this variant, ignore the Strength column of the Armor table in chapter 5
Normally variant rules do not completely negate racial or class features. So how is the dwarven trait supposed to work in this case?