Dear Kori,
yes the file exist, but it's in a ASM.
When backup_type=online_mirror parameter is set, the brbackup create a filelog.spa directory, copy on the file header and then catalog in primary_db. Seems brbackup is not looking into the ASM, but in filesystem.
I do not know if the backup_type=online_mirror type is supported with +ASM?
Thank you,
Jorbson Oliveira