isc-dhcp and wds

[NiKiZe]

If you have multiple DHCP servers on the same network then that is bound to cause issues...
If you by "WDS server" only mean proxy dhcp then you could possibly add

Code:

option ipxe.no-pxedhcp 1;

see https://gist.github.com/robinsmidsrod/4008017

also I would recommend that you do not use user-class to detect iPXE, see the example linked above for a more stable approach.