Previously working builds have suddenly stopped being able to resolve ${hostname} for workspace name

with the following settings in the job cfg
>> Workspace (client) ms_${hostname}_evo11main
>> Client name format for slaves ms_${hostname}_evo11main

previously this resolved to ms_build5pcw7_evo11main
now it is resolving to ms_43_evo11main
where 43 is the first part of the machine ip address (43.xx.xx.xx)

nslookup works find in both directions for jenkins server and slave.