Our daughter who was 17 during the tax year is now not counted as a full dependent. Is this an error or a new tax law change?

Your 17 year old is still your tax dependent.   But you lost the child tax credit in the year that she turned 17.  You get the $500 credit for other dependents instead.  That has been the law for quite awhile, but sometimes it takes parents by surprise. when their first child turns 17.  If you qualify for earned income credit you still get EIC for her.     You can still enter medical expenses for her, and if she becomes a college student someday you will be able to claim her until she is 24, and get the education credits.  So she is your "full" dependent.  But you will not get the $500 stimulus money for a child over 16.

https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents

A 17 year-old is indeed able to be a dependent for the full year of 2019.  You may be confusing dependency with eligibility for the child tax credit or stimulus.  To be eligible for either of those, the child must be younger then 17 at year-end.