@libreoffice LO latest version as it was installed only last week: 25.8.0.4
The proton drive file was corrupted. The LO ODT files on my mac just simply were never created... Despite me hitting 'apple S' multiple times yesterday while working and despite saving and closing the document before I upgraded my OS.
I'd love to have brought this up with your support site link from the programme itself but the link doesn't even resolve...