If 2 taxpayers married during 2018, one had health insurance through a work plan and other through marketplace, why does PTC stop as of month married they paid all year?

While I don't know the particulars of your income, the Premium Tax Credit has income limitations that which may have affected your ability to claim the credit on your jointly filed return.  For a two-person household, your modified adjusted gross income cannot exceed 400% of the federal poverty level, which, in 2018, is $65,840.00 (https://familiesusa.org/product/federal-poverty-guidelines) In addition, the credit is only it is only available to people without another offer of affordable and adequate coverage; in most cases, this will mean that people with an offer of employer-sponsored coverage, such as coverage available through a spouse's health plan at work, will not be eligible for the premium tax credit.