Comment 26 for bug 1677684

Not intentional at all... It seems derived from the not existence of that Breaks since Focal and removed when applied the same fix.

Searching where the Breaks was added (as it isn't present in Debian's versions), I found it added due to LP #1740892 for Xenial (2.3.5-3*),Artful and Bionic (same debian version for both, 2.4.2-3*), i.e :

corosync (2.4.2-3ubuntu1) bionic; urgency=medium

  * Properly restart corosync and pacemaker together (LP: #1740892)
    - d/rules: pass --restart-after-upgrade to dh_installinit
    - d/control: indicate this version breaks all older pacemaker, to
      force an upgrade of pacemaker.
    - d/corosync.postinst: if flagged to do so by pacemaker, start
      pacemaker on upgrade.
      + This can be dropped after bionic releases as long as the other
        changes are maintained.

 -- Nishanth Aravamudan <email address hidden> Tue, 30 Jan 2018 16:40:31 -0800
Available diffs

It seems that the right thing would be to recover the Breaks.