I have tried this in 3 different versons of Maya and everytime it works fine (2016, 2018, 2019 and 2020).
I select the faces for one half, "Edit Mesh->Detach" and then move the faces. Now my lines(Edges) for the object are blue and the faces grey and selected faves are redish in color. If the lines are green and you have all grey faces you have the whole object selected.
If you are still having trouble you could try
1) "Edit Mesh->Extract" - Now this will seperate the object into 2 parts.
2) You can then move the one half to were you need
3) Now select both parts and then go "Mesh->Combine" . This will mean the "Bridge" will now work as its one object again.
I hope that helps - No idea why its not working for you.
There is always the good old fix for many Maya problems of deleting the Maya folder within "My Doc`s" when Maya is close. These will be restored when restarting Maya, resetting all your settings to defualt.
Last edited by tweetytunes; 11-05-2020 at 10:58 PM.