Most beneficiaries will get the full COLA increase in 2019, but there are still a number who are not paying the full Part B premium because of the hold harmless provision that applied in 2017. That’s the part in the law that does not allow an increased premium deduction from a SS check if the COLA is not at least equal to the premium increase.
If you are not paying $134 in Part B premium (ignoring income based premiums), part of your 2019 COLA may be used to bring your premium up to the full 2019 premium.
… there are still a significant number of people — roughly one in four — who aren’t yet paying the full amount. To the extent that past payments are less than the eventual premium amount that Medicare will calculate soon, Social Security checks won’t necessarily go up by the full amount of the COLA. Only once that hold-harmless reduction is fully accounted for will full COLAs apply on a net basis.