On the shares system seats especially made for infants do not show up as an I ..Like on the codeco system which would have be easier for the check in agents if it was on there and displayed.
Problems like these have been pretty much ironed out at check in and staff are getting into the full swing of things.
If pax check in online and select their own seat with an infant then that is not Flight Supports fault. Of course if asked and the seat was an error but rectified in time we would do what we could to allocate them the seat they needed with minimum fuss and disruption. For instance if they wanted to clarify the seat they selected was suitable for an infant before proceeding through security. This rarely happens as we only see those who have checked in online if
1) They get confused and still think they have to check in
2) Need to fast bag their luggage
Average joe pax think a seat is a seat (I would) but we in the industry know different.