Operating system: Windows 10
Graphics card: Nvidia 1070 (8gb)
Short description of error
Custom properties on the armature transform export and import fine, but on the bones they are ignored. I spoke with Bastien in blender.chat and he mentioned that this is a TODO that may have fallen through the cracks.
Exact steps for others to reproduce the error
Attempt to export this simple armature (attached), deform bones only, with custom properties. Import it back to see that the properties are not maintained.