simulator model editor: conversion from old fields format not working

Description

Mature fields values:

Expected behavior: model DB3 (Value field) in library ${SPICE_LIB_DIR}/diac.lib used as subcircuit.

Current behavior: model editor unable to convert mature variables, no model assigned.

Steps to reproduce

KiCad Version

Application: KiCad Schematic Editor

Version: (6.99.0-4760-gc326c57e05-dirty), release build

Libraries:
	wxWidgets 3.0.5
	FreeType 2.11.1
	HarfBuzz 2.7.4
	FontConfig 2.13.1

Platform: Linux 5.15.0-56-generic x86_64, 64 bit, Little endian, wxGTK, cinnamon, x11

	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.5.1
	Curl: 7.81.0
	ngspice: 31
	Compiler: GCC 11.3.0 with C++ ABI 1016

Build settings:
	KICAD_SPICE=ON