So I've been discussing about him with KingPepe and we made a what could have happened to that guy.
So first of all, he was probably working for Sachen, YEH appear 2 times. Once as Marico YEH and the second time as Jack YEH. A bit after in 1991 he is credited in Magic Bubble as Jack Yeh.
In SF2, there's a third programmer which is named "Gordon JAK". Is it the same Jack ? Maybe.
The background of MF6 (Yoko) has been reused in SF3. Also some people have noticed that some SF3 stuff is similar to the some Cony graphics (as seen on the forum). there's also a YEH credited in almost all Cony games (mainly David YEH). And one Cony game has the exact same label as MF6. It's not a bad picture, the colors match the original one and the YOKO-Y1 has been replaced by 60. The original picture is still behind the 60. Nothing was used to hide YOKO-Y1. Which means they had the original artworks. Also the color of Master FIghter VI' text has been modified to orange but the design still match up perfectly with the other one.
I think Jack Yeh left Someri Team when they were done with Master Fighter VI' and took some actual graphical stuff that he could reuse. Then he would have made SF3 and World Hero. Then he created/joined Cony Soft.
That is the only explanation I've found to explain the similarities with Cony/Yoko/YEH