Collabora works with ODT docs but will not save PDF

I’ve got Collabora setup on another server and everything seems to work except I cannot save PDF documents. The Save As option is greyed out and when I try using the Save Comments option, I get the warning Document cannot be saved. Check your permissions or contact the storage server administrator. I’m not sure why the permissions would be incorrect if I am able to edit a ODT file in the same folder (root) and then be able to save it without incident. I’m using version 21.0.0 on CentOS 8 ARM (maybe that’s the problem?) with the Collabora Office setup on CentOS 8 x64 running as a docker container.

The relevant error from the Collabora server seems to be:
wsd-00006-00044 2021-03-16 21:56:47.061256 [ docbroker_001 ] ERR Cannot store docKey [/index.php/apps/richdocuments/wopi/files/631_oc961j38hjyu] as .uno:Save has failed in LOK.| wsd/DocumentBroker.cpp:1028
wsd-00006-00044 2021-03-16 21:56:47.061291 [ docbroker_001 ] ERR error: cmd=storage kind=savefailed| ./common/Session.hpp:136

Shouldn’t the ODT save work the same way?

Just for kicks, I fell back to an older docker version and saw a different behavior. It still didn’t work properly, but actually got a little further. This is obviously a bug. The document features work great, but the PDF annotating and downloading just doesn’t work at all.