A little more googling and it looks like there's an issue with 10.12 Sierra where mount point ownership changes to root/wheel.Review /var/log/samba4/log.smbd and post any relevant information from when access 'drops'.
For what it's worth I took a look in log.smbd and all I could see were thousands of lines like the following:
[2017/05/21 21:02:42.788846, 0] ../source3/modules/vfs_fruit.c:986(ad_read_rsrc_adouble)
ad_read_rsrc_adouble: invalid AppleDouble resource Software/Virtualization/.AppleDouble/VirtualBox-4.2.18-88780-OSX.dmg
[2017/05/21 21:02:42.810583, 1] ../source3/modules/vfs_fruit.c:640(ad_unpack)
invalid number of entries: 13
[2017/05/21 21:02:42.810639, 0] ../source3/modules/vfs_fruit.c:986(ad_read_rsrc_adouble)
ad_read_rsrc_adouble: invalid AppleDouble resource Software/Office/.AppleDouble/TextWrangler_4.0.1.dmg
I couldn't see anything else obvious. Is there a particular search string I can use?