Would someone from ESRI please tell us if MapImageLayer will eventually support Arcade expressions (and therefore TextFormatting.NewLine) via LabelClass?
Currently LabelClass labelExpressionInfo : { expression: .... } is only an option for FeatureLayer, but I really need it to be available through MapImageLayer in order to have multiple field and multiple line labeling. Otherwise, the only workaround to my knowledge is to create a duplicate FeatureLayer from a MapImageLayer's sublayer, add it to the map, hide feature graphics, and then apply labeling. This is not a viable solution.