Why can't my knee fully extend after my ACL recon?

I had an ACL reconstruction 29 years ago using my middle patellar tendon as a graft. However, even after all these years, it's still not able to fully extend. There's like a 6° bend in my knee. I'm still able to go long-distance jogging, squatting heavy weights, and I can bend it, but it's not as bendable as my non-surgical knee. I've been in big trouble when I over-flexed it doing the yoga "hero pose."

Anyways, I was wondering what's impinging my knee from extending all the way out like my non-surgical knee. Is it something in the FRONT of my knee that's blocking full movement, or is it something associated with the tendons and ligaments in the back that's unable to fully relax (or has been made shortened since my ACL reconstructive surgery 29 years ago)?