The F/A-18C Early Access Guide state on page 387 the following behavior for FLIR and AZ/EL page:
This behavior works as described when the tracks being "TDC released" on are onboard tracks (visible on your radar).
Attempting to perform a "TDC release" on an offboard track still partially works; doing so boxes the "SLAVE" text and the track is "remembered". However, the TPOD does not track properly. If the track is picked up by radar and becomes an onboard track, the TPOD snaps to the correct point and behaves.
This behavior can be seen by switching a track back and forth from onboard and offboard via radar slew or silencing the radar.
This behavior appears regardless of when the track is "TDC released" meaning you can start with either an onboard or offboard track and the same result is seen.
The attached track file shows the behavior (PG map, ATFLIR, DCS version 2.7.18.30348)
In the track you will see multiple AZ/EL page "TDC release" actions onto both onboard and offboard tracks. Behavior of FLIR is shown to snap on and off tracks as they switch from onboard to offboard and back. Notice the "SLAVE" text boxed when a track is "remembered" and has been properly "TDC released" on.
EDIT:
Just a note of why this feature, when working, is so cool: Using the FLIR pod in this way is almost like having an IRST but with the "S" replaced with your DL. The FLIR tracked air functionally works for a lot of stuff like AIM-9 aiming, keeping a DT2 track while in STT, etc. You can use datalink and FLIR to be sneaky and track targets without turning on your radar, maintain tracks when the target drops off radar in a notch, or launch a FOX-1 while keeping an eye on his wingman via the DT2 track.
flir_azel_page_not_slaving_to_donor_tracks_when_offboard.trk