Hi, just adding my observations to this issue - I have experienced similar boolean problems (in all Affinity v2 products [on windows for me]) - it does seem to boil down to one object being subtracted from text that was converted to curves. (Even text that was converted to curves via Designer/Photo v1). An easy repro is to take, say, a big letter P, convert to curves and then create a square, angled at 45 degrees counter clockwise and overlay the top right of the P ; subtract the square from the character and the whole thing vanishes.
I can also confirm using the exact same steps in Designer/Photo v1 work like a charm.
I also noted that the new Shape Builder also has similar issues identifying the overlapping regions to boolean when one of the shapes came from a converted True Type glyph.
Hope it helps track things down...