My first set of fasteners had a high failure rate. About half had their head pop off while either removing them from the support material, or with only mild torque while using them. I had been using standard print settings, and found that they worked better when printed at highest fill and print quality. But many still sheared off more easily than I found acceptable. I've found a solution.
The gray bolts are the original style; the gold are new and improved. Notice that the gold countersink head has a continuous slope, where the gray one is disjointed. On the button head bolts, the gold has an undercut at the head interface, while the gray head is flat and just "stuck on top".
On the other hand, when the shaft of the undercut bolt starts to print, three outlines are printed, with two fill areas. This makes the print have more structure, and better adhesion. When I tried breaking a "flat" bolt, the head popped off easily. When I tried breaking an undercut bolt, it was considerably more difficult, and it snapped where the threads start, rather than at the head. In the photo above, you can see the sheared head of a "flat" bolt, and a nice set of "undercut" bolts. Looking closely, you can see the slight groove on the head, where the undercut is printed.
And thank you, TeamTeamUSA, for your suggestion to print them at a 10° angle. As you said, this causes the printer to "outline" the entire skin of the bolts, covering all the fill. Perfect! :)