where is your Syslog Server, does the Loopback 0 can able to reach the syslog server ?
Firewall Interface means, that need to to belong to inside or outside or mgmt...so on.
can you post the configuraiton to look. give syslog IP also.
check loopback interface and limitation :
https://www.cisco.com/c/en/us/td/docs/security/asa/asa919/asdm719/general/asdm-719-general-config/interface-loopback.pdf