I have Libre Office version 18.104.22.168 and just tried to print a DL envelope and had no problems.
When I prepared the envelope, I went into Format/Page and selected DL Envelope. then Landscape. This set the paper size. I then typed the envelope with a return address on the top right side and the recipient address amid-ships. Put the envelope in the printer, long ways in and bingo.
As a matter of interest do you have to change the paper size in the printer properties? I didn't on mine. I also have to say that I am using a MAC, but the principles should be the same on a Windows machine.