VISA Serial Bytes at Port update rate

Using VISA serial bytes at port on a cFP-2220 device I noticed that the reported number of bytes did not update sufficiently often during a continuous transfer. It updates, but only when a rather large chunk of data has been received (which is more noticeable to us now because of a low baud rate):

Here is an example:
Bytes at port is only updated when a certain chunk size has been received

In this case there is a continuous stream of bytes (well, I am really just assuming that as it is sent as one write operation...) coming in on the port at 1200 baud (so about 2 characters per 20 ms), and as soon as the reported number of bytes exceeds 0, the value is rechecked once every 20 ms and plotted.

I have been expecting the number of bytes to increase quite gradually in such cases (approx. 2 bytes/sample), but it stays stable until a sizable chunk of new data has been transferred. This erroneous assumption has not created any problems earlier simply because we have been using much higher baud rates

Does anyone know if this is always the case (does the chunk size vary depending on the hardware for example, are there any settings that affect it etc.), and if this behavior and the logic behind it is documented anywhere?