i dont know if this is a bug, but:
i do some test, i repeat the same healing skill on the same target whit different healing Boost (HB)

"Healing light"
whit 0 HB i heal 1481
whit -150 HB i heal 1279 (-202 from 1481)
whit 358 HB i heal 1962 (+481 from 1481)
whit 499 HB i heal 2152 (+671 from 1481)
1,34 hp x HB

"Healing light Bolt LV3"
whit 0 HB i heal 4632
whit 159 HB i heal 5302 (+670 from 4632)
4,21 hp x HB

in conclusion
Ancient manastone HB +6 in the best case is +24 hp healed

wearing full set Brigate General whith
28 manastone +6 = +168 HB
+14 manastone 70 heling boost +3 = +42 HB
give +210 HB, in best case +280/+840hp healed.... but you become a 10k hp healer)

if that is not a bug, can do be possible considering to remove the HB manastone like the MP manastone?