German engineers didn’t have anything to do with these engines.
Only time German engineers helped Hyundai (before acquisition of Kia) was work on their diesel engines. They messed up big time and Mercedes got involved to save the day. It was still crappy engine from performance/consumption perspective, but didn’t grenade like Toyota diesel engines at 40-60k miles.
BMW engineers, particularly No. 2 from M division, wasn’t engine guy, but suspension guy. That is why best thing on G70/Stinger is suspension, everything else is crap.
As for engineers in general, they change companies. Hyundai has big R/D operation in Frankfurt. That is bcs. access to prestigious technical universities and workforce.
Now, I would really like to know how are H/K engine close to German? Opel for example went down not bcs. it was independent, but too dependent since 1981 on GM. That is approximately when decline started.