Button to print record on a different form

 If you head into your Table settings (gear icon in top-left) and then click on Forms, you'll see a list of Forms. On the right side of the list you'll see the "Form ID" whose value correlates to the value of the "&dfid=" in your query string.

The only funkiness I've encountered with the Form IDs is the default "Main Form" always has an ID of 2. And then, the first new form you create in a given table will have an ID of 10. Then, each new Form ID will increment as you expect (11, 12, 13, etc). That numbering system has been in place for years and hasn't changed, even with the "New Style" Forms.

So, I don't think QB "changed" anything … perhaps you are thinking of a different Form?

Try heading over to your "App Management" page and take a look at the "Manage App Structure" section where you'll see and link to "Search & replace in schema." If you follow that link and then search for "dfid" you may find instances where you've linked out to custom Forms in you app.