Steps to reproduce:

Create a new project with the Universal 3D template on macOS
Create a Cube (GameObject > 3D Object > Cube) and select it
Create a Material (Assets > Create > Material)
In the Inspector, expand the Cube's Mesh Renderer > Materials list
Drag the new Material from the Project window onto "Element 0", release the mouse button and keep the cursor still
Observe Element 0
Set Element 0 back to its original Material (undo) and repeat steps 5-6 about 5 times

Actual result: In about 1 of 4 attempts nothing happens when the mouse button is released, although Element 0 was highlighted as the drop target. The Material is only assigned when the cursor is moved afterwards. Until then the drop stays pending, with no feedback.

Expected result: The Material is assigned on every release

Reproducible with versions: 6000.7.0b2, 6000.7.0b6, 7000.0.0a5, 7000.0.0a10

Not reproducible with versions: 6000.0.85f1, 6000.3.27f1, 6000.6.6f1, 6000.7.0b1, 7000.0.0a1

Tested on (OS): macOS 27.0.1 (M1 Max)

Notes:

Also happens when dropping onto an Inspector array header, and when dropping a Prefab from the Project window into empty Hierarchy space (no instance until the cursor moves).
Windows is not affected