[Table] Adding column/row next to multi-merged cell breaks geometry and content

[FilipTokarski]

📝 Provide detailed reproduction steps (if any)

1. Go to all features test
2. Add a table
3. Create one big cell merged from several others
4. Select some cell next to this big merged cell and add a column

✔️ Expected result

Empty column is inserted

❌ Actual result

Insertion breaks table geometry, content jumps from cell to cell

When you then type something in the big cell and add a row above or below, you get this:

📃 Other details

You can play with this in different cell configurations. Also happens with multicell selection.
Similar issue: #5882

• Browser: any
• OS: any

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[Reinmar]

DUP of #5882.