When I began mortising green 8 x 8 posts for my Shaker Shed (above), I found that it resulted in significant wood-checking overnight, emanating lengthwise from the mortise. I wonder if that didn't happen to your post below.
My theory was that the deep mortise allowed the post's center to start drying too quickly, creating great stresses. My solution was to swab out the interior of the mortise (and sometimes the surfaces of large tenons) with water-based polyurethane as soon as I was done cutting. It never happened again.
This is my third TF (it's kind of an addiction) and I've pretty much concluded that unless you're using UV dried timbers (think big microwave oven and a lot of $$) you're going to get checking. The Oak posts were cut ~2 years ago and air dried in a tarp tent, the checking had already started when I cut the joinery.
I try to mitigate the checking by hitting the end grain of the timbers with Anchor Seal right off of the saw mill, and then I treat them with 50/50 BLO/mineral spirits on all faces after completing the joinery. I hit the inside of the mortises with the 50/50 on the long grain and Anchor Seal on the end grain... and they still checked some.
I much prefer to work with pine. But this project has ~60% of the timber coming off our lot from trees I cleared to make way for my COVID garage in 2020. Then an ice storm took out the big Oak you see me cutting (it was split in two at the first branch about 18' up).
I have to do these projects on the cheap, which means I work with what I have and do as much of the labor as possible myself.
