[6.x]: Unable to add matrix blocks within matrix field

What happened?

Description

I created a "Body Content" matrix block with a "Body" matrix block, however when I add the first Body Content block, it just keeps spinning and won't let me add any body blocks at all. This breaks only when I choose the View mode to be "blocks", when it's "cards", it works as expected. Unfortunately it doesn't trigger any errors I can see in a console or network tab

Steps to reproduce

1. Create a matrix within a matrix field and set both fields view mode to be "blocks"
2. Add to an entry type
3. Try to add blocks

Expected behavior

Block are added as per Craft 5

Actual behavior

CMS won't let me add any blocks

Craft CMS version

6.0.0-alpha9

PHP version

8.5

Operating system and version

macOS 26.5.1

Database type and version

MySQL 8.4.6

Image driver and version

No response

Installed plugins and versions