Not sure where it came from, but many people tend to say Yisbarach after the Amen Yehei Shmei Rabbah verse. Where did this come from? Is this correct?
The question should be the other way around. The Mechaber (O.C. 56:3) says it is forbidden for the answerer to interrupt between almaya and yisbarach.
The Magen Avraham writes that the implication of the Bais Yosef is like Gershon wrote, but sides with the old custom of stopping given the importance of listening to the chazan specifically.
Our minhag is like the GRA to only go up to almaya to get 28 letters, not necessarily because yisbarach is separate. Although our nusach which has yishtabach with a vav seems to indicate that yisbarach is separate.