It would appear that in the html api forms the choices for HyperlinkedRelatedFields went missing from version 3.1.3 to 3.2.0. Let me illustrate with 2 screenies:

Form in 3.1.3:

Form in 3.2.0:

serializers.py: https://gist.github.com/SafPlusPlus/dc3b70e5270a3f708edf
views.py: https://gist.github.com/SafPlusPlus/cd25f6871f74586fb34c
models.py: https://gist.github.com/SafPlusPlus/8087fa8916b7e6a1ed26