I'm having the same issue running v1.14.3 and PHP v7.3.18. I can try testing the latest develop branch tomorrow, but here's what info I can provide at this point.
I've tried in both Firefox and Chrome, and both show errors in the console after clicking the "Add a ticket" button from either the Merge Tickets or Link Tickets modal on a ticket detail view.
Uncaught TypeError: ticket_id is undefined
onclick https://XXX/scp/tickets.php?id=604:25
If I set a breakpoint, I can see this is coming from the inOriginalIds = ids.includes(ticket_id.toString());
line in include/staff/templates/merge-tickets.tmpl.php