1. Create table on confluence page
2. Click into first cell and insert text "Foo"
3. Press Tab key to advance into next cell
4. Write text "Bar"

Result:
Editor shows "Foo" but not "Bar" but when page is submitted or previewed "Bar" is displayed as well. When when going back to editing from preview "Bar" is still inaccessible in editor.

Interestingly if you press Tab at the end of a table, the editor will add a new row and there you can use Tab and write text as normal.