All the PIs that I had informal interviews with except one got back to me with formal interview invitations. And I am seeing in the results that the program where that one PI is is already extending their formal interviews..
I had one informal with a PI last cycle, and she got back to me to let me know that I wasn't chosen for formal interviews before she submit her list to the department. So I'd say it also depends on the PI. I definitely find the one that got back to me much nicer, and I applied to the PI again this year, and got a formal interview.