Looked like a minor issue, but was due to a more serious bug, when using 2 different DSN_LEXERS which were not synchronized.
The fix is not perfect, but unfortunately, the parser used to read the PCB_PLOT_PARAMS in .kicad_pcb files is also used in legacy board file reader.
Therefore it is better than write 2 parser functions, one for legacy files, the other for the .kicad_pcb files, which make the same thing.