Dapper not closing connection - aspnet core/Oracle

I'm making some tests with Dapper, but there is a strange error.

Even with "using" statement, if I don't call the database for some time, e.g. 5 min, the next call gives me a "not logged on", the next call works.

I have created a scheduler that calls the DB every minute and had no error, if I did the same, every 5 minutes, it gives me the error.

I think that the connections is still open in the pool, even if I call an "open" , "close" connection and "using".

The connection's creation:

protected IDbConnection Connection
{
    get
    {
        var oc = new OracleConnection(DatabaseConfig.ConnectionString);
        oc.Open();
        return oc;
    }
}

the method:

public async Task<List<ItemLista>> GetItemListaAsync(int idLista)
{
    using (var connection = Connection)
    {
        string nomeProcedure = "GC.PCK_ECOM_LISTA_PRESENTE.ILC_LER";

        var parameters = new OracleDynamicParameters();
        parameters.Add("E_TX_GC", dbType: OracleDbType.NVarchar2, size: 4000, direction: ParameterDirection.Output);
        parameters.Add("E_TX_TS", dbType: OracleDbType.NVarchar2, size: 4000, direction: ParameterDirection.Output);
        parameters.Add("P_ID_LISTA_LIC", dbType: OracleDbType.Decimal, value: idLista);
        parameters.Add("R_REF_CUR_ILC", dbType: OracleDbType.RefCursor, direction: ParameterDirection.Output);

        var result = await connection.QueryAsync<ItemLista>(nomeProcedure, parameters, commandType: CommandType.StoredProcedure);
        //this connection.close changes nothing
        connection.Close();
        return result.AsList();
    }
}

Asp.net-core 2.2

Dapper 1.50.5 (can't use 1.60.1 cause I'm using Dapper.fluentMap, and it's not compatible yet)

Oracle.ManagedDataAccess.Core 2.18.3