He’s one of the highest earners at Bayern and doesn’t have an agent so it might genuinely be a case of looking for a new challenge whilst he’s still got 4 years or so at the top.
Bayern lost some big players for next to no transfer fee over the years. Likes of Kroos, Alaba, Lewa, Thiago etc.
Barely recouped 100m between them. So next summer could the time to get him for a cut price if it looks like he won’t extend.