System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce RTX 2080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.59

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: rBf1aa4d18d49d
Worked: (optional)

Short description of error
We have an Addon where you can set up your project path in the Addon preferences that uses a StringProperty with the subtype='DIR_PATH'.
When editing the text field by hand by for example pasting in a path, the userprefs are written at blender closure.
But if you select a path with the directory browser the userprefs are not written on closure.

Exact steps for others to reproduce the error

• Have an Addon with AddonPreferences class.
• Have a StringProperty of subtype 'DIR_PATH' stored in the AddonPreferences
• Open a empty blender scene and open addon preferences.
• Set the StringProperty using the the directory browser button next to the text field.
• Close Blender
• Open Blender again, now the StringProperty is empty.

Thanks!