Known limitations of Multiple Forms
The Multiple Forms plugin is still in beta, which means that some native GravityView functionalities that usually work on a simple View still don't work on a View with multiple joined forms.
Here's a list of known limitations from the current version
1) Can't specify fields for editing
Since each joined form has its own link to edit entries, it's not possible to specify which fields can be edited. The Edit Entry tab must be kept empty for entry editing to work.
2) The Edit Entry link must link to each form
If you wish to allow entry editing, a link for each form must be added to the View.
3) Merge Tags from Joined Forms
It's not possible to access or use Merge Tags from other joined forms inside a Custom Content that belongs to a specific form.
To use Merge Tags from a different form, you must add a Custom Content field from that other form:
4) Advanced Filters support
Only fields from the main form are supported on the Advanced Filters
5) Merge Tags on the Edit Entry tab
Merge Tags added inside a Custom Content field on the Edit Entry tab won't be parsed.
6) Impossible to edit Nested Form fields
GravityView supports editing Nested Forms fields, but if the View has a joined form, editing a Nested Form field doesn't work.
The Edit Entry page on a View using Multiple Forms:
7) It's not possible to specify a field from a joined form in the Search Bar
While it's currently not possible to add individual fields from a joined form into the Search Bar, the search will work if the "Search Everything" option is added to the Search Bar widget.