Since survey settings inheritance have been introduced, empty attributes have always been inherited.
But for some attributes, an empty value is actually a valid attribute.

In those cases, the inheritance rules get confused. Is the attribute empty because it should be inherited or because it is deactivated?
Created specific rules for specific attributes.

PR: https://github.com/LimeSurvey/LimeSurvey/pull/2370

I thought in this case the field would store the value 'inherit'.

I really think we must move inherit to null in database.
Then you can merge attributes
finalValues= array_merge(
$globalValues,
$groupSettings,
$surveySettings
);

Maybe we can reproduce with some array_filter ?
array_filter($surveySettings, function($setting) {
return $setting != "inherit";
});

The solution I uploaded is a temporary solution.
Alternatives we thought:

Right now the buttons are "Inherit On", "Inherit Off".
Make them "On", "Off", "Inherir".
In case they select "Off", we can:
a) store "N" in the email field or
b) store "N" on a new field.
c) store empty on the email field.
That would be a UI complement to the current patch.
Not much else needed.

I think the best and the less error prone is alternative B.

I really think we must move inherit to null in database.

Not sure I follow. Isn't that a different discussion?
The problem here is that we are using empty as a NO option but the inheritance is understanding it as an Inherit command

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=33445

Fixed in Release 5.3.12+220502