hello GT Merkley

your post is very interesting, and one that I believe is fixable with an inserted new tenon

It sounds to me like the original tenon on the end of the horizontal has been broken, rotted or sheared off

I have repaired just such problems by preparing a tenon of the proper size to fit the existing mortise, sliding it through the mortise in the vertical post from the exterior face, and continuing to slide it on into a new channel chiseled into the end of the horizontal timber, of course the same size and thickness of the new tenon, and I am suggesting a length of 16 " into the end of the horizontal timber for good stability

If there is any rot forget about it at this point

once in place pull the whole unit together with what ever means you have, so that the horizontal fits neatly into the seating on the vertical post, once in place bolt or use 1.5" wood pins to secure the new tenon in position--5 through the horizontal and 2 through the vertical no doubt the old holes in the post still remain, if so use the size originally used

This type of repair using wood pins, will not be noticeable and will effect a repair that might even be stronger than the original attachment

I hope this helps

You should use experienced help to facilitate the repair

NH