On Downton.....Michael & Mary were the big love affair, but after just 2 seasons, Michael (actor) wanted out to be able to do other projects. So, after having his wife give birth, he is happily on his way home, when a truck crashes into his car, he is ejected, and left sprawled on the roadway. The next season began 6 mos. later. Fortunately, was just a couple seasons into the show, but Michael was a huge favorite. Yes, the show told a lot of stories at the same time, so they could concentrate elsewhere. As it was, Mary's sister had already died in childbirth.
And I do agree, Hallmark could have had Jack away for several weeks, returning with a recast. And of course, Elizabeth is now pregnant. I guess the show thinks that "little Jack" will ease the viewers' minds. Not mine!