Why doesn't CLI work when GUI is loaded?

CLI command ‘libreoffice --convert-to pdf untitled.odt’ only works when no other instance of LO is running. Is there a way to run LO from CLI with more than one instance present?

This is either a duplicate of, or at least answered by, How to NOT connect to a running instance, which also includes a workaround solution.

this looks like a bug to me

you can register it here: https://www.libreoffice.org/get-help/bug/

Reopened: https://www.libreoffice.org/bugzilla/show_bug.cgi?id=41686

Yes, it is a bug. What you can use instead is unoconv, which communicates with LibreOffice for doing bulk conversions. It also allows to instruct the import and export filters (e.g. to select specific pages to export to PDF, or import files in Unicode) and can iterate over files in a directory so it offers far more flexibility for bulk conversions.

Unoconv works on Linux, MacOS X, FreeBSD and Windows.