Version: 5.2.7.2
Build ID: 1:5.2.7-1+deb9u5
CPU Threads: 2; OS Version: Linux 4.9; UI Render: default; VCL: gtk2;
Locale: en-US (en_US.utf8); Calc: group
Debian 9 with Xfce 4.12.
Start with new document in Writer. Select Insert>Envelope. Filling in addressee and sender. Go to Format tab>Size and select #10 Envelope. Go to Printer tab and see first orientation is selected. (This is a horizontal layout from top left corner.) Verify correct printer.
Instructions do not say what New Document versus Insert does, so select either. Print the page.
It will print vertically on right side of printer tray. Totally wrong!
To make matters worse, it will print thru the document and then end up with the top left envelope characters (return address) cut off and missing. There still is plenty of paper edge left to go, so that is a bad template format.
LibreOffice Writer envelope printer tab.png
LibreOffice Writer as laid out on screen.png
LibreOffice Writer as envelope came out of printer.jpg
By the way, I just selected a different orientation and it made no difference on printer output.