As of commit #92cdab8bf8ea1a72e1745b88e9e31b59560e6f7b from the Github mirror, documents are only opened as read only. I believe the issue arises here: 1 -> https://github.com/LibreOffice/online/blob/master/wsd/DocumentBroker.cpp#L436 which leads to 2 -> https://github.com/LibreOffice/online/blob/master/wsd/LOOLWSD.hpp#L88
@Aron, could you please confirm this?
Moving it back to UNCONFIRMED until someone else confirms it.
I tracked down the bug to the actual WOPI host sending the BaseFileName parameter in the WOPI CheckInfo response as just the filename without the file extension.