What can I say, other than we have very different experiences. Opening an .ics file from the Files app indeed shows its details, but selecting "Done" does nothing but dismiss its contents and adds nothing to the calendar.
Contrast this with opening the same .ics file from an email attachment or viewing it online, which presents an "Add All" button.
Contrast this with opening the same .ics file from an email attachment or viewing it online, which presents an "Add All" button.