Comment 12 for bug 1766217

Update to my previous comment:

When the updates for Linux 4.15.0-34-generic became available (but before installing them), I retried some of these tests, and could no longer reproduce the problem.

I re-added each of those problematic /var/lib/apt/extended_states entries in turn; in each case, update-manager continued to display the pending updates as it should do.

(I didn't think until too late to try re-adding all of the problematic entries at once -- but then, that hadn't been necessary previously; in my earlier round of testing, any one of them had sufficed to break update-manager.)

Between installing the backlog of updates (as mentioned previously) and then installing quite a few new packages (all user-land stuff -- build dependencies for digikam), I have no idea what perturbed things enough to make a previously solidly reproducible bug go back into hiding. (With the possible exception of kde-pkg-tools, none of those many installs and upgrades was to anything obviously relating to packaging itself.)