Hi Andrew,
There is an open example in HFSS on Ferrite circulator (both uniform and non-uniform magnetic bias). From menue go to File>Open Examples...>HFSS>RF Microwave>Ferrite_Circulator.
I suggest you first go over that example and check your settings accordingly.
Also, please review the help document on "Ferrite Permeability Tensor in HFSS" to double check your setting. You may search for this keyword in HFSS online help.
Also, instead of creating two separate airbox at the two end, delete them both... Then right click in the modeler window, Create Open Region. Select Rad boundary (which is the default), select the x and y paddings to 0, and in +z and -z, you may assign it accordingly.
If this does not fix the issue, please create a suppoert request through customer portal to investigate the design file.
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