IBB removes blocking constraint
The IBB algorithm to compute Backbones (org.sat4j.core/src/main/java/org/sat4j/tools/Backbone.java) removes it's blocking constraint in every iteration (Line 214) which leads to the learned clauses being flushed indirectly. Without the line, the next blocking clause would subsume the previous one, so I think that that line is unnecessary.
Otherwise, is there a unit test case where flushing learned clauses is required to not induce invalid backbone literals.