Thanks for the responses.
I did check for something loose because that's what it sounded like at first. When I didn't find anything I started to rule that out by trying different acceleration and deceleration combos/severities that should have made a difference in the sound if it was something loose. And then when I found it doesn't happen in sport mode I think that confirms it's not something loose in the car/spare area or even a loose bolt or something in the suspension.
I don't see a jump in RPM on the tach...it seems to hold steady through the thud then drop off shortly after but maybe that is how the low gears respond on this 8 speed. Tried a few times to count the upshifts then downshifts to ensure it was still in 2nd before the thud but haven't yet found myself in an isolated enough situation to be able to get to a high enough speed and focus completely on the tach to not lose count. Will try to find a wide open space in the near future. If it is just the last downshift then maybe the adaptive transmission will work it's way out over time or can see about resetting it to let it start fresh.
If I hear from a few others like cv27 that seem to indicate this is normal for this car then I'll probably be able to just tune it out over time...or spend more time in sport mode