Redmine

hi!

I am trying to understand the expected behavior for list custom fields with multiple values enabled.

Example:
Custom Field: Flags
Format: List
Multiple values: true
Possible values: a, b, c, d

Issue examples:
1. Flags set to a, b
2. Flags set to a, c
3. Flags set to b, c
4. Flags set to a
5. Flags set to b

Filter example @1:
on Flags, condition "is a, b".

Result of query:
Issues: 1, 2, 3, 4, 5

My Expected result:
Issues: 1

Instead of providing explicit result (AND), the result is all issues that have a OR b present in their list.
If this is the official expected result, is there a feature that can control the logical operator for the list of values?
How i can achieve the expected result?

Condition "is not" is also tricky :).

cheers