Arcanist turns into both Scholar (a healer) and Summoner (a DPS).

Scholar uses gear with Mind on it, and physick scales off Mind, so their physick is useful at level 80 (well, not saying I use it much at level 80).

Summoner uses gear with Intelligence on it, and since physick scales off Mind, it does nothing at level 80 because you have almost no Mind.

People who have played the game for a long time are well aware of this and so are the developers, yet they've kept it this way. It is strange so maybe they will do something about it at some point.

The reason a 400 potency heal can do less than 400 healing is because the maths works something like this:

Mind ÷ Divisor x 400
e.g. 289 ÷ 3333 x 400

which results in almost nothing. It's more involved than that and includes weapon damage, substats and trait healing boosts but that's why.