“A Delta package for this update will not be available. Customers using Delta package updates need to apply the Full Update. Customers who do not apply the Full update, and only the December Delta package update from December 11, 2018 will experience an update failure when installing the January Delta Update. The January Full update will install correctly.”
December 19th update was also a security update released for a major vulnerability (although no delta update package was generated for it). January 2019 Delta was calculated against December 19, not December 11 update.
Usual logic of selection whether delta or full package should be installed on specific computer was changed for January 2019 Patch Tuesday updates – delta package is selected for installation if computer was updated on December 19th, full package is selected otherwise.
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