Yes, this makes a lot of sense. Updating the trusted SSL certificates list on my local machine is making all the difference. Backups to B2 are working again!
This is indeed the same issue and solution as in Restic stopped connecting to backblaze.
For those with Synology DSM, here are the commands I used, based on b2: Failed to initialize gateway with error x509: certificate signed by unknown authority and Import root certificate in Synology.
openssl s_client -showcerts -connect api.backblazeb2.com:443 > /usr/share/ca-certificates/b2.crt
ln -s /usr/share/ca-certificates/b2.crt /etc/ssl/certs/b2.crt