First, take anything I say with a grain of salt. I am no where near a profession, just trying to help.
What chuck (name/size) do you have? What size/type jaws.
I have a Nova SN2 mounted with their PowerGrip jaws. Your blank size exceeds their capacity and for myself I would never even try it.
Are you trying to turn it end grain or side grain orientation?
With either I do not understand how the tendon can break. I can only assume you are turning side grain (normal bowl orientation) and the wood had a weak place. You said it is green so it should not be punky (rotten). Even with a proper fit to the tendon you are chucking up where the grain of the wood allows the tenon to break off.
With my SN2 and PG’s the rating is for end grain orientation up to 8” dia # 12” long. (like gripping a soft ball bat in the jaws and trying to break it). It does handle larger…up to 27” diameter I believe…BUT that is a max of 2-3” deep as in a table top or large platter.
Again, I am just trying to help so if I sound out of line excuse me. The president of our local turning club had an accident (last month or the month before). I didn’t hear all the details of what he was turning but a squished nose (broken in three places) and a broken cheek bone ain’t any fun.
I’m glad the tenon broke before you could mount it and bring it up to full rpm.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety. Benjamin Franklin