If the gpupdate doesn't work on the first attempt, it will pretty much never work after that (for that boot) Of the 30% of the time where a problem presents itself: The problem is mostly random, and does not seem to follow any particular user or Workstation. Sometimes it is both drives, sometimes it is one or the other.

dcdiag /test:dns /v /c /e reports PASS for ALL servers and ALL tests.As part of Group Policy, two network drives are mapped to shares on the Synology server.Each site has a local (on-site, same subnet) Synology storage server.Both networks are fully gigabit running on Cisco switches with certified CAT6 cabling.ALL clients are Windows 10 Pro 64-bit with all updates.DNS records for each site ONLY have the two local DNS servers defined.Sites are correctly defined in Windows Sites and Services.Each site has 2 local (on-site, same subnet) Windows Server 2012 R2 Domain controllers.