First, retired Polish Archbishop Jan Pawel Lenga, said that bishops who force people to receive Communion on the hand are going to “end up in Hell”. HERE
For a person to wind up in Hell he has to have committed a mortal sin. For a sin to be a mortal sin for which one has culpability, the matter has to be grave, the person has to know it is wrong, and then go ahead and do it anyway, or else counsel it to be done, force others to do it, etc. There are NINE ways to share in the guilt of another.
Everything having to do with the Eucharist and Communion is serious, so sins involving the same would be grave because of their matter.
As far as knowledge is concerned, bishops today are men of their age. It is likely that some of them had really bad seminary formation. However, they spent time as priests, during which time their priestly vocation obliged them to inform themselves about all things central to their vocation. Bishops must do the same. Priests and bishops cannot easily claim ignorance as a defense when it comes to knowing that certain things are sinful. Their ignorance is culpable.
Furthermore, it is beyond the authority of an individual bishop to prohibit Communion on the tongue. And yet some do this anyway, with the excuse of “public health”. They simply get away with the ultra vires imposition of their will because the flock has so little power to fight back.