The purpose of the Tank Mastery trait is to transform you from a DPS into a Tank. In order to achieve this, your health and mitigation must be increased and your damage must be reduced.

So it is strange then, that the English translation of the Tank Mastery trait implies that damage is increased. The people who math out the damage in this game know that is false. The Strength attribute increases damage less on a tank, yet...

Reduces damage taken by 20%. Furthermore, grants a bonus to maximum HP based on your vitality attribute, and a bonus to damage dealt based on your strength attribute.
The original Japanese version implies no such bonus to damage dealt, according to Google translate.

Reduces your damage by 20%. In addition, the tank roll-specific correction is applied to the amount of increase in maximum HP due to the increase in VIT and the amount of increase in damage inflicted due to the increase in STR.
So the Japanese version is just saying that there is an appropriate adjustment for the Tank role to maximum HP and damage dealt. It does not say one way or the other whether this is an increase or a decrease, but it implies that HP is increased and damage is reduced, as explained at the start of my post.

When translating, no doubt in the rush of things to translate, this was interpreted as an increase to both attributes, but it has been incorrect for 3 years now and is misleading. It's minor, but should there be some time to carefully rephrase this, it will remove a noticeable translation anomaly from the game.

It could be as simple as replacing the underlined "bonus" with "reduction".