OK, thanks for the replies.
The version of the drawing where the text disappeared was one which had been edited (in model space only) some time ago, and it was only one of the paperspace tabs where the text went AWOL, other tabs were fine.
Pleased to see I have a unique problem around here!
Another one I will post when I can replicate it is where an object, starting out as an extrusion or a 3D shape, is edited by slicing or subtracting another 3D object, and then suddenly becomes uneditable.
You can still select it and Selection Info
tells you that it is indeed a TC Surface
and everything looks normal, except when you want to further slice it or subtract a shape from it, and then no amount of clicking can select the object.
I have the feeling that it occurs when I have carried out a large number of edits on an object, but that may just be my perception.
I will play around with a test object to see if I can replicate it and post it here.
I know TC Expert is a scaled down version of the Platinum Pro, and am considering upgrading to this, so it would be interesting to know if these issues are unique to Expert.