Hello @christoph-gd,
My name is Henry Eduardo and I am happy to assist with your inquiry, and appreciate you contacting LastPass Support!
I am sorry about the issues you are having with the shared items. Due to how LastPass works, when logging in to a site LastPass enters the password of the item you shared. Within LastPass, the other users will not be able to see the password. However, once the password is outside of the LastPass environment some users may find ways to retrieve it. Please see the Security section on our Sharing Center article.
Please let me know if this worked or if further assistance is required.
Thank you,
Henry Eduardo | Customer Care