The mouse pointer turns into a "Not Available" icon but it's hard to notice this change and also even if you notice it then the message is not clear. I've tried to understand what was going on a few times before realizing it.

The link to the old ticket which has been merged into another one should be removed from the extension list.

Btw I'm also linking tickets from different departments. It may be working for merged tickets from the same department but in our use case we use the linking feature for tickets on multiple departments.