Net class definitions live in <project>.kicad_pro (net_settings) on KiCad 7+, not in the .kicad_pcb that the SWIG board save writes -- so create_netclass mutated the in-memory board and reported success, but nothing survived a reload.
Write the class definition and its net memberships (netclass_patterns) into the project JSON, which is what KiCad reads on open. The transform is a pure, unit-tested function that needs no live KiCad/SWIG round-trip; the in-memory NETCLASS path is kept for live-session consistency.
