I don't believe that this is working as intended because it was working this way in 1.12.2 but not in the 1.13 snapshots. You are able to place a torch if one is in your left hand if you have any other item (or no item) in your right hand but if there is a block in your right hand, you cannot place the torch with your left hand. Some people, including myself, have used this as a feature and does not make sense to have taken it away.
I don't believe that this is working as intended because it was working this way in 1.12.2 but not in the 1.13 snapshots. You are able to place a torch if one is in your left hand if you have any other item (or no item) in your right hand but if there is a block in your right hand, you cannot place the torch with your left hand. Some people, including myself, have used this as a feature and does not make sense to have taken it away.