The part of _descend_and_update starting from this line appears to be dead code:
https://github.com/pixos/poptrie/blob/master/poptrie.c#L929
Since this code returns immediately, the rest of this if statement is not executed.
I think this could be a bug that happens when you try to add prefixes of greater than 24 bits from this line:
https://github.com/pixos/poptrie/blob/master/poptrie.c#L880