Reproduce 1:

• open diagram with empty cells
• type something in the cell
• undo by Ctrl-Z

Expected 1:

• cell is set to empty

Observed 2:

• cell is not set to empty

—

Reproduce 2:

• have a non-empty cell and delete the content
• undo by Ctrl-Z
• redo by Ctrl-Y

Expected 2:

• cell is empty

Observed 2:

• cell is not empty

(Both may have same cause. Possible rendering issue, unrelated to the underlying model)