It appears in latest builds of GZDoom seeker missile functionality is completely broken when used on Weapons. A_SeekerMissile doesn't work, but the Revenant tracer functions does.
I tested this with the latest GZDoom - g2d0a876ab.
It seems to break around ge63f3e394, with some of the MBF21 changes.
ga9ad3d1fc works fine.
https://www.dropbox.com/s/1tbn1wha98v9e ... t.pk3?dl=1
Use the rocket launcher. It will give you a tracer-enabled seeker missile.
EDIT: Looks like someone else is having problems with it.
viewtopic.php?f=2&t=72804