|emily (Remember The Milk) says:|
At this time, the different modes for things like due date and tag entry can't be used in combination (same thing for due dates; if you change in the Picker view and then switch to the Custom view without saving, it won't use the new value).
While we can look at ways to improve this in the future, the fastest way to do this currently is to set the existing tags first, tap save, go back into the Tags field, and in custom add your new tags.
You can also create new tags before associating them with tasks (by going to 'More' then the 'Tags' screen, tapping 'Edit', then the '+' button).
Hope this helps!
Posted 6 years ago