Darkness Enthroned interaction

Not sure if intended, the 1% mana reservation implicit on abyss jewels doesn't have the 50% additional effect of socketed jewels applied to it. I have 2 socketed in my darkness enthroned and I only receive 2% reduced mana reserved instead of 3%

Belt doesn't "buff" the "sum of both jewels".
It counts each one separately and rounding in PoE betrays (pun) you.

1% + (50% increased effect) = 1,5% => rounds to 1%.
Same for other jewel.