I thought that according to the work of Ma et al. 2012 (https://onlinelibrary.wiley.com/doi/10.1111/j.1365-294X.2012.05684.x) and the interpretation of Defaut et al. 2013 (https://www.researchgate.net/publication/283151818_L'etude_de_phylogenie_moleculaire_realisee_par_MA_al_2012_sur_le_Criquet_migrateur_nouvelles_conclusions_taxonomiques_Caelifera_Acrididae_Locustinae ) the taxon cinerascens was more related to L. m . migratorioides than to L. m. migratoria. There is still some work to carry out to clear this case up but it would be interesting if there are additional arguments to justify this swap.

Fair enough! Still I hope more integrative work will be carried out soon to clarify the taxonomy of this genus in Western Europe.

You were right @mathieu_pelissie. It's a mistake to synonymize Locusta cinerascens sensu auct gall. avec Locusta migratoria migratoria. As I wrote in other place, Defaut is very clear in its 2012 paper : "Précisons d’emblée que ce n’est pas le vocable migratorioides (Reiche & Fairmaire, 1849), adopté par les auteurs, qui convient à la lignée méridionale, mais le vocable cinerascens Fabricius, 1781, car il correspond au taxon de cette lignée le plus anciennement décrit (article 23.1 du C.I.N.Z.) "
The name cinerascens was not used till its resurection by Defaut, and the synonymy Locusta migratoria cinerascens = Locusta migratoria migratoria is not the one used recently in France and consequently in Spain (I don't know in other neighbouring countries). The OSF interpretation is a mistake and I try to change it but I have no response until now.
For southern France, the true synonymy is Locusta cinerascens auct. gall. = Locusta migratoria migratorioides