Did you add any yeast nutrient at all? Everything I have read about Kveik is that if there is one factor that will limit it, it's nutrient availability. Honey is pretty sparse when it comes to nutrients, when making mead it's a must (pun not intended) to add some for example in some form. Could be it just ran out of micro-nutrients before it was able to finish munching on the macro-nutrients (the sugars). I tend to chuck 5mls (1 tspn) of Tronozymol in 5 minutes before the end of the boil for this reason. I know you don't need it with normal yeasts, but Kveik breaks all of the rules after all....
That said, comparing mash temp with attenuation, I've been seeing this:-
69 degree C mash (quite a few dark grains) : 78.5% apparent attenuation
67 degree C mash (no dark grains): 79.5% apparent attenuation
even with extra nutrients... I think 80% attenuation is maybe the highest you'll get, rather than what you will definitely get... That's with Yeast Bay Sigmund's Voss Kveik.
As to braggots coming out thin. The one I did I used Belle Saison, which has a strange ability to produce glycerol as it goes, which gives your beer body (you can glycerol to "age" wine and mead for the same reason). So you get a super dry beer, which isn't actually thin. It's a bit boring other than that though as yeasts go. The yeast formed a solid layer on the bottom of the bottles though, and is now peeling off in sheets when I serve it....
This is why I use glass bottles now, not PET....
Oh, and the golden ale I made with kveik, well odd one this. Dunno if the yeast esters are there, but what is there is far more hoppy aroma and flavour than there should be... I used 4g Admiral at 70 minutes for bittering, and 35g of Jester at 15 minutes for aroma and flavour, and the aroma is bonkers! Serious hempy notes (my wife goes nuts if I call it dank....) on the nose. Oh, it's a 14 litre batch (so ok, thinking about it, it's probably the equivalent of a lot more hops in a bigger batch... lmao).
Just bottled the chocolate & mint porter too. I lost 4.5 litres, somehow the tap came unscrewed on the FV and leaked out....
Tested a small sample after adding the essences, delicious! Not too much mint at all, and you could taste the chocolate nicely (with the vanilla giving the chocolate a nice roundness). Just thanks to the leak I only had 9 litres into bottles, rather than about 13.... My wife was a serious trooper, capping for me even though she has a really bad back! She put on her TENS belt and got the job done. She will be rewarded with the lions share of the porter naturally (she gets most of the beer anyway, as she should. I enjoy watching her enjoy my beer more than I enjoy drinking it myself. lol).