Undo fails after changing root attribute few times

[FilipTokarski]

📝 Provide detailed reproduction steps (if any)

1. Set root attribute to foo: A
2. Set root attribute to foo: B
3. Set root attribute to foo: A
4. Undo
5. Undo

✔️ Expected result

Undo in step 5 should show foo: A.

❌ Actual result

Undo in step 5 still shows foo: B.

📃 Other details

• Browser: …
• OS: …
• First affected CKEditor version: …
• Installed CKEditor plugins: …

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