In 2021, #317 was made and merged (and included in 0.2.15.0). This changed the branching factor / bitmap mask size from 16 to 32.

This was updated in the module documentation of Data.HashMap.Lazy, but not in the module documentation of Data.HashMap.Strict. There it still states:

Many operations have a average-case complexity of (O(\log n)). The implementation uses a large base (i.e. 16) so in practice these operations are constant time.