I suspect your tree has verticillium wilt - the staining of the cut branch is pretty characteristic - but you would need a pathologist to confirm. If it is VW, there is no treatment and nothing you can do. It will struggle on as best it can but it will eventually die.

Can't tell by the pictures and don't know you location but I tried growing a red maple in a very sandy location. After a couple of years it broke off in a wind storm. The inside of the trunk was pulpy, porous like yours but didn't have the staining.

Maples are generally water loving plants, they don't tolerate dry conditions very well, maybe better once an established tree.

Many of the maples grown around here seem to take the dry spells but sometimes years later, show damage induced by the sometimes season long droughts.

Your climate and soil may be quite different from mine but if it isn't, the stress of too little water can send a tree like that into decline and makes them susceptible to various other maladies along the way.

Just an FYI. :-)