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Hello Alan Medows,
The combination of text disappearing while typing and paragraphs unexpectedly becoming bold does not seem to fit normal text prediction or Overtype behavior, so I would not want to assume those are the cause.
Could you check File > Account > About OneNote and share the exact Version and Build shown there?
Also, as a quick isolation test, could you create a completely new page, insert a new simple table, and see whether typing there shows the same behavior? This would help determine whether the issue follows the current OneNote build or is specific to the existing page/nested tables.
If the issue also reproduces in a brand-new table, that would be useful information to include when reporting it through Help > Feedback in OneNote.