Great, that's what I figured.

Your underdrive clutch's clutch volume index (CVI) has climbed too high. The only time you can feel the underdrive clutch come on while driving is a 4-3 downshift, which is why this is the only time you feel it.

Several things can cause this. The root cause is a loss of fluid prime. This can happen when doing work to the trans, if the fluid goes low from a leak, or failure in the trans that causes delayed engagement when cold (pump, low/reverse piston, etc.).

With your very low mileage it's obvious that the car sits for extended periods. As the car sits it will lose fluid prime naturally and there's no way around it, other than periodically starting the car and letting it run. In your case what most likely happened is it lost prime while sitting and that caused the CVI to climb high.

In any event, what needs to be done to fix the harsh downshift is a quicklearn. Quicklearn is done with a scan tool and sets the CVI's back to a baseline to allow them to learn correctly again. This just takes a few minutes to complete using a capable scan tool.