We set pegasus.gridstart.arguments = -Z

For clustered jobs, we get this in the .in file:

#@ 1 mProjectPP:3.0 ID001250
pegasus-kickstart -n mProjectPP:3.0 -N ID001250 -R ec2 -Z -L montage ...
#@ 2 mProjectPP:3.0 ID001253
pegasus-kickstart -n mProjectPP:3.0 -N ID001253 -R ec2 -H -L montage ...
#@ 3 mProjectPP:3.0 ID001254
pegasus-kickstart -n mProjectPP:3.0 -N ID001254 -R ec2 -H -L montage ...
#@ 4 mProjectPP:3.0 ID000121
pegasus-kickstart -n mProjectPP:3.0 -N ID000121 -R ec2 -H -L montage ...
#@ 5 mProjectPP:3.0 ID001251
...

So it looks like pegasus.gridstart.arguments is replaced with "-H" for jobs 2-N, instead of being added to the current pegasus.gridstart.arguments.