I am using Coral 3 Tagfield on AEM 6.4.5 with multiple and required set to true. However, after submitting the dialog error message pops up saying "Please fill Category Tag". Dialog is successfully submitted.It works fine for single value tagfield.
Its look like, there are custom validation is written for your component. That is showing popup.
OOTB required validation highlights the field if invalid like below:
You should check the logic written for validation in custom code.