Okay, I finally went to my parish priest with this. To my surprise, he told me that I wasn’t the first one to complain. So I asked him point blank if it’s “wrong” or just a violation of convention. or what.
He pulled out a book, something like Roman Missal: The Ceremonies of The Mass, and showed me that in the formation of any priest, these were the rubrics that are to be followed, laid down by the Catholic Church. It spells out very clearly that the right hand only is to be used to offer Communion (along with many other restrictions, such as keeping the thumb and forefinger together except when obtaining another Eucharist to offer to the next communicant, so any particles fall back into the Chalice, etc).
He said he didn’t personally witness this happening, so there wasn’t much he could do, but if that priest should ever be back at our parish, he would monitor him. The most interesting thing he said was that the only reason a priest would offer Communion in the left hand was out of direct disobedience. No priest anywhere in the world is instructed otherwise, he said.