I was unable to reproduce this issue in a current Xenial installation, so it's possible that there was a correction to the Xenial package sometime in the past couple of years.
I am going to mark the tmux->Xenial portion of this bug report "incomplete" for the time. If someone knows with certainty that this was corrected in a Xenial package, please update this bug report with the relevant information.
I was unable to reproduce this issue in a current Xenial installation, so it's possible that there was a correction to the Xenial package sometime in the past couple of years.
I am going to mark the tmux->Xenial portion of this bug report "incomplete" for the time. If someone knows with certainty that this was corrected in a Xenial package, please update this bug report with the relevant information.