Just realized that under ‘Events > Settings’ for Ticket Settings > Post types that can have tickets, the Option for ‘Product’ page has been removed.
We had to deactivate Event Tickets Plus and Event Calendar then select ‘Product’ there again.
Then reactivate the plugins to see Tickets published on Product pages.
However, when we revisit the setting ‘Events > Settings’ for Ticket Settings, the post type ‘Product’ has disappeared again.