[3.2.16] Calendar/event : External URL field disappears when not logged in as adm

[charled]

Hi,

I have a calendar - let call it mycalendar - and I want some user to access nothing else in BE but this calendar.
So I created a user group - let call it mycalendargroup - to which I granted these authorizations :
- Calendar permissions : mycalendar
- Allowed fields : Title and some other fields, Redirect target, Redirect page, Article, Link target.

When I edit an event logged in as an administrator, I see : Redirect target > Redirect target : Use default, Page, Article, External link.
When I'm logged in with mycalendargroup permissions I only see : Use default, Page, Article but not External link.
Do I miss something (another checkbox not checked) or is there a bug ?

I've tested with clean 3.2.16 and 3.4 installs, I still have the problem. I've tested with english and german backend : same problem.

Thanks for your help.