Need serious help with subnetted DHCP issue across Metro E...

Happy Sunday /r/networking I'm at my wits end with this issue that popped up ~6 weeks ago out of the blue. Years of running this same setup and one night, issues. I'll be brief:

Quick background: 2 offices. Cisco SG300 core router and SG200 switch at office 1; Unifi Switch at office 2. Unifi access points throughout both. 4 subnetted VLANs, Msft DHCP 2012r2 servers. All networking hardware at office 2 was replaced and issue prevailed (originally another SG200)

All users at Office 1 have zero issues retrieving an IP address no matter which network they join.

Many (not all) users at Office 2 suddenly are receiving DHCP timeouts. ISP says there have been no changes but the night this popped up they took us offline for 6 hours for maintenance. Many hours on the phone with them, they essentially say "not our problem, it's your network" and stop assigning engineers to it.

Here's what i'm seeing:

• Windows PC joins any of our wireless networks at office 2, no issues. Full DHCP communication takes place and quickly gets an IP. Quick sequence of events:
  • Device sends a DHCP request across the Metro E, it's relayed and the DHCP server sends the Offer back.
  • SG300 relay sees the offer and sends it across the Metro E to the client
  • Offer arrives at office 2, DHCP completes its process (request, ack) and client is happy
• Mac, iPad, iPhone, some Androids join any network at "office 2" and nothing. Self assigned IP address. So I trace the packets, mirror ports, etc:
  • Device sends a DHCP request across the Metro E, it's relayed and the DHCP server sends the Offer back.
  • SG300 relay sees the offer and sends it across the Metro E to the client
  • Offer never arrives at office 2, as seen by mirroring the incoming port, TX/RX packets.

If I assign these devices static IPs, they work great. ISP says there is no way that they are dropping a single packet out of the DHCP stream and nothing else. Not their problem.

Next step was to compare an offer packet for the 'good' device and the 'bad' device. These packets look great except for one thing (as far as I can tell), the Broadcast Flag:

• The 'good' devices are requesting and receiving a broadcast Offer packet. All of my dropped packets seem to have the Unicast flag set in both the Discover and Offer. These packets clearly leave on the Metro E port from office 1 and never arrive at office 2

How would this be possible? What can I do to further troubleshoot?

I tried the IgnoreBroadcastFlag registry setting, no dice. Happy to post the packets, etc.

Thanks in advance...