Hi Laura!
There are a couple more places for you to check to ensure that Workfront is calculating timelines as expected.
Within Setup there is an option to select how the task should respond when multiple people are assigned to it.

And then within the individual Project Setting (also can be set at the template level) you can say if individual schedules should be taken in account as well as the schedule for this project.

Check these settings on your project to make sure nothing is contradicting each other. And then play around with the assignments. Workfront should in fact not count any holidays when planning a project timeline. If you notice that the dates are accurate before you make assignments, then I would look at the individual's schedule as well as the User Time Off setting within the project settings.
Also, make sure that you don't have any fixed date constraints on your tasks as that will override the holiday and time off scheduling features. For example, if you have a project that must finish on September 4th that will be the due date regardless of it being Labor Day.
If you still run into trouble, please share screenshots of the schedule as well as the projects and we can help troubleshoot this further. For me personally, it was the multiple people assigned to a single task that was throwing off the scheduling.