I guess im surprised that no one has encountered an issue with filters not showing accurately. In Photoshop I do not have to think about what magnification I am at. The software will figure out a way to show me how "much" my filters are affecting my image and I can tweak it to my liking. For example noise and such.
In affinity since it is live, i guess, only a 100% magnification is correct and if you export the image thinking it will look the way it did when you worked on it, on lets say 25%, youre going to have an exported image not looking like what you had in front of you at the time. Same thing happens as you merge. So the fix is quite frankly to merge often to see how the effect will look in the end and adjust accordingly it seemd.
Still for me this is a deal breaker and I have to experiment a bit to see if there is a way to work with it.