Here are a couple of Screenshots:
In the diagram settings, the preview looks OK.
In the export wizard, the whole board looks OK, the diagram is missing the horizontal lines.
Dito for the SVG.
Any hints/help would be greatly appreciated.
Post scriptum: the images I tried to insert have not been displayed. The Links are:
https://imgur.com/zeNNEMZ
https://imgur.com/hmMKrhh
https://imgur.com/rZzZvYs