At first I thought it was a graphical glitch. So I closed Revit and Re-opened. Same problem. So I restarted my computer. Same problem. Opened the project on another computer, same problem!
I was intrigued! This was my kind of puzzle.
After a little searching and a whole lot of head scratching, it turned out to be a 'face palm' moment, when I realised the workset I was working on, was set to not be 'visible in all views' (it wasn't supposed to be!)
What puzzled me about this problem was that, whilst I knew text objects were automatically given a 'view' workset in Revit, I never noticed that initially, those text objects & the leader seem to respect the current 'user' created workset settings while they are being made. Interesting? I thought so.
Once the text is finally placed, it takes on the uneditable 'view' workset that Revit automatically assigns.
This eventually led to a team members guilty confession and highlights the importance of communicating with your team, especially when it comes to global changes in the model.
If you do need to temporarily hide worksets in views, use the Temporary View Properties. You can read more about it here.
Finally, it is worth mentioning that, despite the 'visible in all views' being unticked in the worksets dialog, if you have another Visibility Graphics settings showing the workset, then you will not encounter this problem.