Current behaviour: On occasion (not all the time) when you tell a cleric to heal, they may not heal. It could be Viconia specific.
I've had this happen to me in a number of different places at this point, but the first time it happened I was in the area just south of Beregost. I had ordered Viconia to heal Montaron, however, she did not comply. I've since found with this bug to expect one of two things to happen. Either a) you get the healing spell symbol shown on the top right of her portrait, or b) you'd get nothing shown. Either way no healing happens in either case. Since then I've found normally if I leave the area or I rest it fixes the bug. However, she did have these spells memorised, so this is a bug.
Expected behaviour: Viconia (or any cleric) should be able to cast any healing spell they have memorised when they are ordered to do so.
It may be a Viconia exclusive bug I'm not certain, I have no other healing members in my party so I can't test it without a new playthrough.