I just tried this, and yes, I can confirm it.
And the bug is that the copy of the receiving track does not receive anything on channel 3. If you set the copied receiving track to have 4 track channels, it works.
The bug seems to be in Reaper that when I do a mono send from one track to another, selecting "New channels on receiving track" and I select the mono channel 3, then the receiving track is not set up correctly to have 4 track channels. In fact, the track channels is blank:
If you manually set up the receiving track to have 4 track channels, then the copying of the tracks works as it should.
This is a Reaper bug, I would say.
EDIT: I think this is better posted as a new bug report, with a more descriptive title.