This was due to a bug in the March cumulative update.
The solution was to disable TFTP Variable Window Extension inside the WDS properties.
I have been reading of many cases where the June CU KB4503276 for Server 2012 R2, which was meant to fix the PXE booting issue is causing a PXE boot issue in WDS/MDT/SCCM.
Solution for 2012 R2:
This has been fixed with KB4512512 on 17/Aug/2019.
Uninstall KB4503276 if you are running Server 2012 R2. Note: If you are using SCCM, you can turn on the native SCCM PXE server: Go to Administration > Distribution Points Open the properties of your DP > PXE Tab Tick “Enable a PXE responder without Windows Deployment Services”. This will take a few minutes and no reboot is required.
I hope this info helps everyone.
Please comment if you have experienced this issue?
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