Section part names appear and disappear when adjusting other elements in that section.

I have a score, where the movements are separated with sections and the "begin each part with a long part name" has been unchecked. None of the parts uses a short name ie short names have been deleted from each stave. If I make any alterations to the positioning of the slurs in the first bar of the second section, for example raising or lowering the end of the slur, then the part name appear. Other alterations to the second part cause the names to disappear, then appear again.

I see a number of issues reported against sections but nothing that quite matches this behaviour. Does this sound like a known problem? I'll try to create an example of this and attach it if no one recognises this problem. Here's an example: section.mscz

Select and double click the slur on bar 9 of the 1st stave of the second section. Raise the right hand handle.