The main FJCC 1964 papar, by Vyssotsky, Corbato, and Graham, spelled
Multics with an initial cap. By contrast, Ken transcribed the aural
pun as UNIX. The lawyers did their best to keep it that way after most
of us had decided it looks better as a proper noun.
As I recall, there was an acronymic reading of Multics, but it wasn't
taken seriously enough to drag the word into all caps. Nobody proposed
an acronymic reading of UNIX. So both words defy the convention of
rendering acronyms in upper-case.
Doug