pretty wierd. i wonder if you'd see the same behavior using zmmailbox to mount the calendar.
if usera is the person who has shared his calendar, and userb is the person you want to have this calendar added to, you can do a command like this on the zimbra server
you might want to get familiar with zmmailbox per the link posted earlier in this thread, it's a great tool for managing this kind of stuff.
zmmailbox -z -m email@example.com cm --view appointment -F# /UserA-Calendar firstname.lastname@example.org /Calendar
see if that has the same issue. if so, youll probably have to file a support request as i've never seen that behavior and we use shared calendars extensively.