Might be the stop-neutral feature from the transmission reengaging Drive. With dirty fluid, they sometimes slam back into gear quite hard.
I'd start with changing the fluid if you feel like that's what it is. You can put it in manual mode, come to a complete stop, and see if it makes the clunk go away (this does not put the car in neutral at a stop)
Just paid 651$ to replace a leaking line to transmission and have it flushed, 450$ to flush tranny and $251.00 to have new line put in!!shifts smoother but still have clunk at stop and yes if I go to manual it doesn’t clunk