Consult with an attorney who will review your loan documents in order to review/analyze any possible modification language.
The money to pay the property taxes are held in escrow and is not to be touched by the bank except for that purpose so they have no incentive to keep changing it.. They do, however, need to have enough money in your escrow account to pay the taxes when they become due. If the taxes are due by 1/31 each year, paying it in February will mean paying it late and incurring a late fee so that makes no sense either. Sounds like whoever did the initial estimate (February payment) was wrong and they are now adjusting to ensure that it is paid on time (January). You do need to pay it because they are using the money to pay your taxes on time. They only "created" the shortage in the sense that they are collecting the extra month now instead of at closing so either way you'd be paying it.