I have an executed separation agreement (April 2018) that stipulates I pay my former spouse maintenance starting March 1, 2018. The divorce decree finalized in September 2018 states the same. When I filed my 2018 taxes, I deducted all the maintenance I paid in 2018 as it was part of a separation agreement and then divorce decree all in that year. My former spouse only claimed as income the maintenance payments he received during the date of the finalized divorce decree and beyond (Sept - Dec). He did this under advisement of his accountant. this makes no sense to me and I assume that this will likely trigger an audit. I plan to ask him to amend is return but I wanted to be sure that I wasn't missing something.
You are correct to include all of the maintenance paid in 2018, both under the separation agreement and the final divorce decree. Your former spouse should also include the total amount as income on his return.
IRS will (eventually) match up the amount you reported on your return with the amount reported on his and contact him.