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Mexico definitely has a VAT tax. And it applies to these maintenance fees.
So either they were not collecting it when they should have been, or they were absorbing the cost themselves in previous years (which is legal).
So you need to look at your purchase and maintenance agreement(s) to see if they have assumed the VAT liabilities in a binding way, or whether they simply were not collecting them before...
Ultimately, if they are in breach of their contract, you can sue to enforce in a Mexican Court.