The answer to this is that technically, yes, the 840C should display 24/48, but only if it sees it as this in the information that is sent to it.
Part of the data package that is sent in the digital signal from a source device should include information about what the bit rate actually is - this 'information' is what the 840C uses to determine what is displayed on its own front panel. Often, many manufacturers do not include this information packet, or status bit, in with the digital output signal.
It is the 'status bit' that requires setting or inclusion in the output device.
There are different levels of status bit - some that indicate whether a file is greater or less than a certain bit rate, and some that go into more detail about what the bit rate actually is.
It would appear that in certain source devices, only the SPDIF status bit that indicates whether bit depth is greater or less than 20 bits has been set. The additional status bits that indicate exact bit depth are not set and hence the SPDIF receiver (eg 740C/840C ) can only report that the bit depth is greater or less than 20/24 bits.
As such, the 740C/840C will be receiving and upsampling the correct 24/48 signal, but can only display 'less than or equal to 24/48', as it has not been told by the source device what the bit rate actually is.