Our organisation is using a brand guidelines document (produced externally) where the hex values and the RGB values of corresponding colors do not match. Both RGB values and hex values use the same color space (as opposed to pantone or CMYK values which have to be approximated in conversions), so I supposed they would match in our document.
For example (recreated for Stack purposes):
These above RGB values written on the color squares (and background color) do not correspond to their hex values.
I often end up with mismatched colors because of shared swatch libraries which are sometimes hex and sometimes RGB, for example when bringing an Illustrator document (whose colors I took from our brand library swatch) into After Effects (where I used the hex code to create a composition's background color).