Turning on/off radar echo trails on chart overlay?

The answer is probably staring me in the face, but I've tried several times and just can't figure this out: how does one control whether to show radar echo trails when overlaying the radar on the chart display?

Obviously it's easy to control whether radar echo trails are on or off on the main radar display, with a toggle setting for that available multiple ways. We have echo trails enabled on the radar display on all 4 TzTouch2 displays on this vessel.

But when turning on the radar overlay to the charts we see one of the 4 installed TzTouch2 displays on this boat showing echo trails while the other 3 don't (they just overlay the immediate radar echos from the last sweep over the chart). Changing the echo trail setting on the radar display doesn't seem to change whether echo trails show on the radar chart overlay. I've compared the settings across the 4 units and they seem identical between the one showing radar overlay echo trails and the ones that don't.

(my goal is to turn the echo trails off on the chart overlay on that one unit too, just like the other 3, as we find trails are useful on the radar but trails clutter the charts a bit too much on the radar overlay).

Can anyone help me with the obvious thing I'm probably missing here?
On the surface; what you describe is not possible. The radar is doing the function not the MFD. If you turn on echo trails it goes on for all listening to that radar. My only thought is you might have two radars on this network. One unit is looking at the second radar while the others are looking at the first. You can check what your radar source is at each MFD under RADAR menu and SOURCE.
Thanks Johnny. Only one actual radar hardware is installed, but just in case I also looked at the installation settings to confirm that the software doesn't show two radars (i.e. somehow thinking the same one NXT radar hardware onboard is actually two different radars) and indeed there's only one radar listed.

I guess this remains a mystery bug then, why the radar chart overlay on just one Tzt2 shows echo trails but the other 3 Tzt2s on the same network don't show echo trails on their radar chart overlays.