Steps to reproduce:

1. Default installation zabbix_agent2, Oracle DataBase, full tablename_space TESTTBL
2. Add second datafile (ALTER TABLESPACE  TESTTBL ADD DATAFILE 'TESTTBL01.dbf' ...)

Result:
 

"TESTTBL": {
 "contents": "PERMANENT",
 "used_bytes": 67914153984,
 "max_bytes": 67914153984,
 "free_bytes": 27746107392,
 "used_pct": 100,
 "status": 1
}

Note: free_bytes counted correctly.

Expected:

"TESTTBL": {
		"contents": "PERMANENT",
		"used_bytes": 40168046592,
		"max_bytes": 67914153984,
		"free_bytes": 27746107392,
		"used_pct": 59.15,
		"status": 1
}

Problem sources:
https://github.com/zabbix/zabbix/blob/69e54b01f32a83ae5c36c3a8d3f311767b541b5e/src/go/plugins/oracle/handler_tablespaces.go

Oracle version: 12.1