When LibreOffice locale is the default one (like Default - English (UK), for example - see the Default part; the setting is in Options-Language Settings-Languages, named Locale setting), then LibreOffice first consults the OS settings to get the default page size (unlike the case when the locale is set explicitly, like English (UK) without the Default part, when LibreOffice consults its in-built rules for defining default page size for given locale).
When looking for OS settings, LibreOffice does that in the following order:
- Checks paperconf (if present);
- Checks environment variables LC_ALL, LC_PAPER, LANG (in that order);
- Defaults to en-US (and thus, Letter).
So, when one has properly set LC_* variables (say, that should result in A4), but one's paperconf returns letter, then LibreOffice will default to the Letter size. This is incorrect system's configuration, and not an error on LibreOffice side. In this case, one needs to consult OS user guide to properly configure the paperconf to resolve the problem.
A workaround is to set locale explicitly, or to setup default template with preset page size.