15 year Japanese Maple has large split and soft wood between broken branch and main trunk.
Growth on 3sides. Noticed this fall. Soft wood fibers in the crevice .
It sounds like the spit occurred some time ago and a fungal infection has caused wood decay. Or a fungal infection caused weakening of the crotch which allowed the split.
I assume your unstated question is "what to do?" But without photos it is impossible to assess and give a recommendation. The general suggestions would be;
1. the tree is now structurally unsound and needs to be removed.
2. The main trunk is structurally intact but depending on severity of the split, the branch needs to be removed.
3. An artificial support system (cable bracing) can save the tree and the spit branch.
4. Soil fertility and water management are important for tree health and resistance to disease and for recovery and wound closure.