Steps to reproduce:

1. Make sure to have some problems on hosts assigned to specific host groups
2. Set up a user with Admin or User type
3. Set permissions to see contents of at least one group, better two
4. Set Tag Filter for one of these groups, with both "Tag Name" and "Tag Value" empty

Result:
Problems which have no tags assigned with them are not shown. Only those that actually have any tag.

In the documentation (https://www.zabbix.com/documentation/current/manual/config/users_and_usergroups/usergroup), it's stated in another way, which also makes more sense as the behavior that currently takes place:

If both tag name and value fields are left blank but host group selected, corresponding user group will see all problems for selected host group

Expected:
As the documentation states, ALL problems from corresponding groups should be visible for user.