If it is turned on ,try with OpenCl acceleration turned off. (Edit-Preferences->Performance). I was getting something similar exporting jpegs and tiffs.
I may have to stop using affinity, as I presently have to export photos I want to print as a tiff file, import into an old CS3 copy of photoshop so I can print. Trouble is when I upgrade to the new MAC OS the old CS3 will no longer work. I either need to purchase a copy of Elements just to print or get the monthly subscription and use it for everything. Affinity does a lot of things pretty well, but lack of color management is such a big negative for those like me that print images that the good features are mostly negated.
