For folks that aren't sure what's happening, this likely means that their grievance process broke down, and the mass filing of grievances is a collective action before a strike vote. I don't know the terms of their contract, but it's very likely that their no strike clause (note, these are extremely common including how I describe it here) has an exception for going on strike if the issue isn't resolved through the grievance.
The other possibility is that the contract breach is an unfair labor practice, which cannot be negotiated away by a no strike clause, so the grievances are telling the company that this is the last chance to abide by the contract.
Edit: it's worth mentioning that if it is a ULP, Stellantis will be unable to hire scabs.