Curiously enough, when around 9 months ago I reported the problem to Epson, the online help fixed it. However for reasons unknown the problem has reappeared.
I have version 7.1.0.3 (x64) on two of my 3 PCs, and 7.2.2.2 on the third. It makes no difference.
Epson’s support seems to be pushing me to blaming LibreOffice:
"Thank you for contacting Epson technical support.
Regarding our recent query about the default document size being “letter” here is what our Level 2 tech team responded:
-Drivers sometimes do revert back to previous settings this usually has to do with the software they are printing from.
-They can create presets in the driver but they would probably need to select them either way.
-Try to print from various softwares to check if it reverts back on all of them."
To which my answer was, no, this problem seems LibreOffice-specific.