HP OfficeJet Pro 8720 Paper Tray 2 Unusable on Linux (CentOS 6.9)

[swdavison]

1. The printer knows about tray 2. I can use either the web server
or the LED panel to set tray 2 as default. Once I have done that, I
can use the LED panel to request a report -- e.g. status report -- and
the printer uses paper from tray 2 as expected.

2. The default tray selection does not seem to apply to copying.
Every time I make a copy I must select tray 2, otherwise the paper
will be drawn from tray 1 even though tray 2 is set as default.

3. The computer software (CUPS with hplip-3.17.4) seems to know about
tray 2. When I install the printer at the CUPS web console
(http://localhost:631) I get the expected dialog about setting a
default paper tray and I can set the default to tray 2 without
getting any error messages. Same thing happens if I use hp-setup for
the printer installation.

But then, with the default set to tray 2 on the printer and set to tray
2 in CUPS, if I try to print a test page from CUPS the printer pulls the
paper from tray 1.

4. When printing from applications (LibreOffice, acroread, firefox)
I cannot use tray 2. That is true regardless of whether I try to rely
on the default tray setting or specify the tray at the time of the print
request.

In LibreOffice, the Properties->Paper dialog has the paper tray
selection disabled. When I open the Properties->Device dialog Tray 2
is set to True and Paper Source is set to Tray 2, as I would expect from
the CUPS settings. But when I tell it to print, it pulls the paper from
tray 1.

The same thing happens, with pretty much the same dialogs, when I try to
print from acroread.

When printing from firefox, the Paper source option is not available in
the Page Setup dialog. But when I open the Advanced dialog the Paper
Source option is set to Tray 2. But the paper is drawn from tray 1
nonetheless.