According to the doc of log format, time_key and message_key are default to time and message respectively. But log info doesn’t contain time and message fields:
{"level":"info","ts":1596024586.9671729,"logger":"http.log.access.log0","msg":"handled request","request":{}}
Config:
log {
format json
output file /var/log/caddy/access.log
level INFO
}