Thanks for the rapid response!

Your checklist:
The TOC styles in the document do have tab settings (if they don't,
default
tabs will be used) and they are correct, including dot leader.
Checked.

The TOC styles are not set to 'Automatically Update'
Checked.

The document is not set to 'Update styles automatically' from
template
(Uncheck setting in Tools Templates and Add-ins)
Ah. It's not.
However, the doc is attached to the a template which resides on the
server, so it's always attached to the same template. Does htis make a
difference?