Steps to reproduce:

1. Open editing of the screen
2. Add in "List of user group shares" "Zabbix users"
3. Change sharing type  to "Public" and Permissions to "Read-only"

Result:
Aftre hit "Update is displayed message"

"

Details Cannot update screen

• Screen "<screen name>" is public and read-only sharing is disallowed.
  "

Expected:
It should be allowed to share publicly R-O to some group screen.