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The nargs='?' option should probably follow the getopt_long(1) convention and only consume an (optional) argument if it's in the same argv element, i.e., without a space. Otherwise, it can only be given as the last option on the command-line. For example, in
./prog -a ARG1 ARG2
ARG1 should not be considered an optional argument to -a, but here, it should:
./prog -aARG1 ARG2

If I understand correctly, this would be a backward incompatible change, so I doubt it will be accepted. Maybe there's some other way to achieve the same end?

Yes, it is incompatible. But that's because the current behavior is 
incompatible with standard (getopt_long(3)) practice. Or perhaps, you 
can add another option that implements the optional argument semantics 
of GNU's getopt_long(3).
In any case, I understand. If you figure out some other way, please let 
me know (or document it).
Thanks much,
Ernest Mamikonyan

I don't think this is going to change.
It's basically a consequence of having a unified handling of nargs='?', nargs='*', nargs='+', etc. These will all consume as many arguments as they can, and argparse doesn't distinguish between arguments that are concatenated to options and arguments that aren't. (And you couldn't concatenate a whole '+' style argument to the option anyway.)
If you really want getopt-style parsing instead, the getopt module is still available. ;-)