VFX Graph can't find custom attributes when they are in a subgraph

How to reproduce:
1. Create a new Unity project (3D URP Editor template)
2. Open up Package Manager (Window → Package Manager)
3. Install the “Visual Effect Graph” package
4. Go to Edit → Preferences ->Visual Effects
5. Make sure that “Experimental Operators/Blocks” is enabled
6. Right click on the Project folder, Create → Visual Effects → Visual Effect Graph
7. Create an Empty VFX
8. In the Hierarchy window, right-click, select Visual Effects → Visual Effect
9. In the Visual Effect’s Inspector locate the “Visual Effect” component
10. Drag the VFX Graph asset, into the “Asset Template” field
11. Right-click your VFX Graph asset to open it’s window/blackboard
12. Click the “+” button in the top left of the screen, Attribute → Float
13. Right-click on the blackboard, press Create Node
14. Search for “Initialize Particle” and choose it
15. Drag the previously created Float into the “Initialize Particle” node
16. Set the “Custom Attribute” value to 1
17. Drag the previously created Float into the blackboard to create a new node
18. Right-click on the blackboard, press Create Node
19. Search for “Multiply” and choose it
20. Change A value to 2
21. Connect the created float’s node output to Multiply (A input)
22. Right-click on the blackboard, press Create Node
23. Search for “Output particle” and choose any option
24. Click on the “Output Particle” node to mark it
25. Press the space bar
26. Search for “Set Size” and choose it
27. Drag from the output port of the Multiply node and connect it to the input port of the Set Size block in “Output Particle”
28. Select the created node and Multiply at the same time (click the created node, hold Shift, click the Multiply node as well)
29. With both nodes selected, right-click on one of the selected nodes
30. Choose “Convert to Subgraph” and save it
31. Reconnect the new subgraph to the “Output Particle” node
32. Save the project
33. Reopen the project
34. Open the same scene
35. Observe the Console window

Actual result: Error: “Exception while sanitizing model: |Set|_Custom Attribute of type UnityEditor.VFX.Block.SetAttribute: System.InvalidOperationException: Could not find attribute CustomAttribute”
Expected result: Unity saves and reloads the custom Attribute correctly

Reproducible with: 6000.0.0f1, 6000.0.79f1, 6000.3.20f1, 6000.5.4f1
Not reproducible with: 6000.6.0b4, 6000.7.0a2
Fixed in: 6000.6.0a6

Reproducible on: Windows 11
Not reproducible on: No other environments tested