It's not good, but only looking at the bad half of a tradeoff will always look bad. Without diving into the glibc source code to understand what it's doing, it is hard to say if this is actually wrong.
I’ve recently been doing a lot of work that involves algorithmic design with non-technical people and this is the fun thing. It’s very easy to identify expected scenarios, harder to identify how to achieve them and sometimes actually impossible on e you consider things like “the algorithm doesn’t know what happens next”.