While Bayern have definitely signed from players from Dortmund before, to me it seems like Haaland is ready to move on to another league, and probably doesn't want to develop bad relations with Dortmund fans.Would not be surprised if the German rags try to get him to stay at Dortmund until the end of next season so they can buy him for the release clause fee. Lewnadowski will be 33 in August so they will need to have a plan to replace him and Haaland would seem the obvious choice.
Hope it does not happen but the pathway from Dortmund to Bayern is well trodden.
Not saying it won't happen, but of all the options likely available to him (I imagine most top clubs want him), going to Bayern would be a weird move. He seems like somebody who wants to experience new leagues.