Win10 / Sierra / Bootcamp - cannot see OS X partition, no CoreStorage, no FileVault
As I do not want to wipe the whole HDD and backup two system partitions, I instead installed Paragon HFS+ for Win10 which ultimately fixed the issue. Give that Paragon merely installs drivers for HFS+ interaction, I tend to believe that the bootcamp drivers must have been affected during one of the Windows Updates, hence suggest others with the same problem to first re-install the bootcamp drivers before turning to Paragon.

The most up to date bootcamp drivers for your apple notebook can be found on the Windows Recovery Drive in the WinPEDriver Folder, you maybe have to assign a drive letter to the partition first (right-click "This PC" > Manage > Disk Management > right-click WINRECOVERY and click "assign Drive Letter").

Hope that helps