Well, I was actually going to suggest this, but you initially stated that they also didn't work where the track voltage was 18 volts, so I rejected that as the cause.
FWIW, I run with Lionel PowerHouse 180 transformers on my layout, and the actual measured voltage on the tracks is around 18.25 VAC everywhere with no load, and around 17.5 VAC when I have the Vision Line Big Boy with four smoke units working and pulling 30 cars. I've never had any coupler, Legacy, TMCC, or MTH DCS fail to operate correctly here. However, on my test bench, if I turn the voltage down to 15.5-16.0 VAC, I can get failures on some couplers.
I would point out that with no load I measure almost exactly the same voltage no matter where I check on my 140 foot mainline or any of the secondary loops. If your 16.5 VAC & 18 VAC measurement was done with no load, that would suggest a possibly poor connection to drop that much voltage with no load.
