According to the doc:

For the '?' format character, the return value is either True or False. When packing, the truth value of the argument object is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be True when unpacking.

But it behaves more like & 1 operation. so False for even value, and True for odd value.

unpack fmt '?H' should reqiure buff of length 3, same as 'H?', not 4.