Electro couplers: Possessed!

Turn your voltage up slightly more.

Simple answer- your ZWL output might read the same value on the meter, but it isn't the same voltage to the coupler.

Again, no, your coupler isn't broken, wrong, or possessed. It's this simple, the spring and the coil are basically matched to need near full 18V AC RMS sine wave power. One track you have enough power to activate and the other you are just below the threshold for those specific couplers. I probably have a dozen couplers in my total fleet that work the same way.

Yes, you could replace them and hope the replacement works with slightly lower voltage.

Or carefully, and within reason, just turn up the track power slightly higher until the coupler activates. Say maybe 18.5-19V. Yes, I know, I'm the same one who tells folks to avoid going over 18V for Lionel products.

Yes Ed, I'd have to agree with Vernon'... just a few volts can make all the difference with the electronics in the locomotives of today....😏

Since the 6-38791 is an RCMC equipped locomotive, the couplers are run on regulated DC, and a couple volts on the track doesn't change anything, they should still fire.

The 6-28361 is an early Legacy with the R4LC and track voltage is important for those couplers, I've found that less than around 16V on the track and you can have issues with the couplers not opening reliably.

Thanks for the advice. I’ll try turning up the voltage.