Caption field goes blank

Using Lr v8.1 from the Mac App Store. 

For some unknown reason (as I've not been able to find a reliable trigger), the caption single line text field just looks like it's gone. 

Clicking where the text field is doesn't do anything.

However, if I click on the field name (i.e. "Caption), then the text field re-appears with any text I've entered into it. 

To resolve the issue, I have to quit Lr to get the text field to be shown. Until it stops again.