Using 'd%' to delete a bracket pair and its contents is now broken

I’ve long been editing code in Vim 7.4 with matchit and matchparen plugins. In my old setup it was easy to delete { some string } by placing the cursor on the { and then use the d% command.

I’ve now moved to a new computer system (and new job) with a fresh install of Vim 8.2 and I believe I’ve got the same plugins, but now when I use the d% command, it keeps the closing }.

This change in behavior now makes editing my code much more cumbersome as I have to deal with the closing } (or [)], etc]) separately.

Could someone who understands the plugins or key mapping please tell me how to get back to the behavior where the terminating character is also deleted (or selected/yanked/etc).