Variable Font Preview Plugin – strange behaviour

The following problem occured:
I can see the variable font preview area on the top of edit view, but it disappears as soon as I move the cursor.
After some research I found out the plugin works as expected as long as there is only one .glyphs document active.
When I open another file it won’t work for this one, whereas it still works fine for the document that has been opened first.
My setup ist glyphsapp version [1330] on mojave 10.14
Does anybody experience the same?