.net - SerialPort Reads Are Returning Fragmented Sets Of Data?

I have a USB device which creates a Virtual Serial Port on Windows. I am using VB.Net to write and read from the port. My device responds with specific sized set of bytes, but I am finding that SerialPort.Read(byte-array,offset,number-bytes) does not return the full number-bytes but it also does not timeout or generate an exception. Repeated calls return additional fragments (up to 3 calls required). Python code using pySerial does not have the same problem.So, what am I doing wrong here? Am I expecting too much?

Some scenarios are:I write a command to the port and expect to get 4 bytes in response. I get 1 byte first and then 3 bytes on the subsequent call.I write a command and expect 21120 bytes in response. I get 1, 12671 and then 8448 bytes in 3 calls to read from the port.

Here are some excerpts from my code:

Private Sub SetupVirtualSerialPort()
Dim portName As String = "COM" + (m_DeviceContext * -1).ToString
Const baud As Int32 = 9600 '7680000
Const parity As Parity = parity.None

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