Good pics!
Yes, your right, yearly close inspection of all hitch equipment is a need.
And as QCman stated, the crack in the dark area started a long time ago and it looks like it may have progressed in stages.
I found cracks in the welds of one of my WD hitch heads. It was about 10 years old at the time. Fine hair line cracks down the center of the welds. Several of them. That is the time to find these cracks, they are tiny and in the early stages of failing. You really have to clean the parts and use good lighting, but you can see some of these issues before they create big problems. The key is, you have to at least look.
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.