after suspend and bg, normally running pipeline job labeled "unknown signal (core dumped)"

When a pipeline job is suspended and then backgrounded, 'jobs -l' incorrectly lists the status of all but the first command in the pipeline.

For example:

% cp /dev/null /tmp/emptyfile && tail -F /tmp/emptyfile | fgrep hello | sed 's/^/received: /'
^Z
zsh: suspended tail -F /tmp/emptyfile | fgrep hello | sed 's/^/received: /'
% jobs -l
[1] + 8096 suspended tail -F /tmp/emptyfile |
       8097 suspended fgrep hello |
       8098 suspended sed 's/^/received: /'

The confusion begins after the 'bg' command:

% bg
[1] + continued tail -F /tmp/emptyfile | fgrep hello | sed 's/^/received: /'
% jobs -l
[1] + 8096 running tail -F /tmp/emptyfile |
       8097 unknown signal (core dumped) fgrep hello |
       8098 unknown signal (core dumped) sed 's/^/received: /'

The status above is wrong; both fgrep and sed are in fact running fine:

% echo hello world >> /tmp/emptyfile
% kill 8096 # flush the pipeline
% received: hello world

[1] + terminated tail -F /tmp/emptyfile |
       done fgrep hello | sed 's/^/received: /'
%

Note also that the final status, printed when the commands actually exit, is correct ('done').

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: zsh 5.1.1-1ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-70.91-generic 4.4.49
Uname: Linux 4.4.0-70-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CurrentDesktop: GNOME-Flashback:Unity
Date: Tue Mar 28 18:58:32 2017
InstallationDate: Installed on 2017-03-27 (1 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: zsh
UpgradeStatus: No upgrade log present (probably fresh install)