Not all migrate messages shown

When on migrate overview page at /admin/structure/migrate/manage//migrations we see a list of migrations and the number of messages.

However when clicking through to the messages page, I am seeing less migrations than there should be on the group migration overview.

Checking in the db messages table I can see the number of migrations on the overview page appears correct.

Looking at other migrations they appear correct, so perhaps this is a particular issue with this migration.

See screen shots attached: