Unable to read file greater than 2K in FatFS
I can't speak to FreeRTOS f_read() call, but for some operating systems, f_read() can only read up the the boundary of the sector size of the underlying media, typically 512 or 2048 bytes. To read more data than that, the f_read() call would need to implement a gather function, reading data from multiple sectors.

We've seen this behavior in nuttx, and I suspect FreeRTOS does the same. To read a larger file, try reading it in blocks sized to your media.