While debugging, I verified that I am writing the attachments to media/local successfully. I also verified that I can access the images via http://homeassistant.local:8123/media/local/<file_path>?authSig=<auth_sig>. Nothing with my automation has changed so I suspect there is a bug in HA companion’s handling of adding the <auth_sig>
I just fixed my issue. I logged out of nabu casa on my HA server and then re-logged in. Afterwards I logged out of my iOS HA companion app by deleting the server (done from settings in the companion app) and then re-logged into the server. I then sent a test notification and voila! Everything is back working.