Racing flow benches
I never said what head it is....... but it’s one that I see this situation occur on often.
It’s a turbulence issue, that can be a challenge to fix.
As a reminder......... these heads didn’t come here for porting work. They came here to correct a seat depth/installed height issue.
They were already ported........ and had there not been an installed height problem, would have been run as-is.