I've added a number of custom profile fields, which only the user, Admin's and 1 specific usergroup is allowed to view.
But as it turns out by checking with a few Regular user's permissions, every user can see the added custom profile fields.

Custom Profile Field visibility (bug)?

I am having exactly the same problem and I have sufficient knowledge in managing permissions in phpbb.

"when I activate these options":

1 Show profile field publicly:
The profile field will be displayed in all available places within the configuration / load parameters. Setting this as "no" will hide the pages field of topics, profiles and member list.

2 Show in User Control Panel:
The user is able to change this profile field from the User Control Panel.

3 Hide profile field:
Hide the profile field from all other users except the user, Administrators and Moderators who can still see this field. If the option to Show on the user control panel is disabled, the user cannot see or change this field and the field can only be changed by the Administration.

Result:
It is no longer visible for all registered user groups, except for administrators and moderators who can see it. But the user himself does not see his custom field here:

/memberlist.php?mode=viewprofile&u=48

But option 3 says that it hides all other users except the user itself. So this seems to be a phpbb bug, I tested it in a clean phpbb installation and the same thing happens.

Making a new test different leaving 1, 2 activated and deactivating option 3 mentioned above

Result:
It can be seen for all registered user groups, administrators, moderators and the user.

Which shows that there is a bug and option 3 is not working hide profile field for everyone except the user, administrators and moderators.

In my forum some custom fields of my users are private and sensitive information, without this option working, any common member can see their custom fields and read their information.

I hope you can help solve this serious problem that violates the privacy of registered users and webmasters cannot respond if phpbb does not solve this problem for us