Had the same issue when fighting Kar'niss; Shadowheart was stacked with healing buff items and casting Mass Healing Word, for seemingly no apparent reason, blew her 2nd turn of concentration on Invoke Duplicity.
Perhaps one of the triggered healing buffs has a bug w/concentration?
Boots of Aid and Comfort - temp HP on self after target healed
Hellrider's Pride - Blade Ward on target(s) healed
Wapira's Crown - heal self after target healed
Ring of Salving - heal target(s) extra after target healed
The Whispering Promise - Bless (2 turns) target(s) after healed (possibly the bugged culprit since Bless spell normally requires concentration—probably left over junk code from the spell version)
