Here's some documentation on that area, from the Help area of ACBC. Perhaps it can help you work through the inconsistencies you are seeing:
How Unacceptables and Must-Haves Are Determined
1. VIA CONSTRUCTED LISTS: If the researcher has specified to treat missing attributes or levels as unacceptable, those are marked as unacceptable for the respondent.
2. INFERRED FROM MUST-HAVE QUESTION: For Attributes with Sequence Order set to "None", if the respondent indicates that one level is must-have, all other levels are marked unacceptable.
3. INFERRED FROM MUST-HAVE QUESTION: For Attributes with Sequence Order, the respondent indicates that a certain level is a "must-have" threshold (e.g. must have "at least" or "at most" this level). Any levels lower (if at least) or higher (if at most) than this level are marked unacceptable.
For example, suppose MPG (miles per gallon) is the attribute in question with levels 20MPG, 25MPG, 30MPG, and 35MPG. If the respondent marks that "at least" 30MPG is a must-have, 20MPG and 25MPG are marked unacceptable.
As a second example, if Seating Capacity is the attribute in question with levels 4-person, 5-person, 6-person, 7-person, and 8-person. If the respondent marks that "at most" 7-person seating is a must-have, 8-person is marked unacceptable.
4. DIRECTLY FROM UNACCEPTABLE QUESTION: For Attributes with Preference Order set to "None", if a respondent indicates a level is unacceptable, it is marked as unacceptable.
5. DIRECTLY FROM UNACCEPTABLE QUESTION: For Attributes with Preference Order, if the respondent indicates that a certain level is unacceptable, that level and any levels less preferred than it are marked unacceptable.
Exception in the case of constructed lists: If an attribute is marked as having preference order, if an "unacceptable" level (omitted due to constructed list) is "more preferred" than the first "acceptable" level, it is changed to "acceptable".