Calc: Formulas suddenly showing Sheet Name

I have a Calc work book which was last opened a few weeks ago. This time, I found that all the formulas with relative references show the sheet name before the reference. Eg Stuff.A3

When I create a new formula, this doesn’t happen. I can also remove the sheet names either by direct editing or by Find & Replace.

Is this a known issue?

Using Mac OSX 10.11.3 and LO (which I notice is slightly dated).

There is bug report with needinfo status:

My main suspicion: A corrupted user profile. Cf. this wiki page.