There was an optimization to skip examination of groups if it was determined
that the group membership had not changed. However, this erroneously
masked the examination in the case of reordered groups. Skip the
optimization to cover that case at the expense of at least some needless churn.
This only happens when something goes to change group membership in some way, so
this shouldn't be too expensive.
Now when expressions can not be completed, the reason is presented as 'broken'.
Additionally, when unsetting a value that would affect expressions,
perform appropriate changes.