I reported a similar issue for 2017, and I’m seeing it again in 2018. I hadn’t noticed it on 18x24 and 24x36 - but will test new page creation with those.
LO seems to be shifting the export.
Screen Cap - purple is the page boundaries. Grey is an inner rectangle for reference.
PDF Output attached.
LO-Glitch.pdf (8.2 KB)
LO File for reference attached.LO-Glitch.layout (9.9 KB)