I don't know why and if this will permanently resolve my issue but I fixed it by changing the transforms.conf file as follows.
Before
FIELDS = month,day,systime,host,month,day,systime,format,time,duration,server_ip,uri_host,clientip,action,bytes,method,uri_path,username,hierarchy,content_type
After
FIELDS = Month,Day,Day,Systime,host,month,day,systime,format,time,duration,server_ip,uri_host,clientip,action,bytes,method,uri_path,username,hierarchy,content_type
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