If memory serves, in standard Nd-B-Fe magnets the neodymium also concentrates at the grain boundary in the same way. My guess is that the Ce/La is acting as a s

If memory serves, in standard Nd-B-Fe magnets the neodymium also concentrates at the grain boundary in the same way. My guess is that the Ce/La is acting as a stabilizer for the internal structure replacing Nd that would otherwise have been stuck inside the crystallite.