holy avenger does not set magic resistance to 50, it increases by 50

i have keldon with holy avenger and neckless of 10percent magic resistance and it sais he's got 60.

Is this a bug? I don't think so, it's just that it increases rather than set.

<font color="gold">Yes, you are correct. Carsomyr has a 50% Magic Resistance bonus, it doesn't set your MR to 50%.
But even if it did, I think the game still would have told you that you have 60% MR. This is because of a bug, where you first equip the item that is supposed to set the MR, the equip the one with the bonus. ;) </font>

i seem to recall that in vanilla SoA Carsomyr sets it to 50%, but that concept is removed by some patch, or was it ToB. or perhaps it was vice versa?

Carsomyr sets your BONUS Magic Resistance to 50%. All other Magic Resistance bonuses are ignored.

However... your BASE Magic Resistance is addable to Carsomyr's Magic Resistance bonus. Base Magic Resistance comes from kit bonuses and certain character changing quests/machines.

This is assuming you did not install the unnecessary mod that modifies Carsomyr to add +50%.