According you inform our blank part hardness abnormal and effected to your cutting tool broken.
HCTD re-checked the actual problem part 3 pcs. (Mizuki return to us).
Microstructure are same as standard of material 18EPC Hardness a little higher than previous lot about 10 HRB (incident part 76-91 HRB , Previous Lot 66-80 HRB)
And the data of hole diameter from Mizuki show the dimension has taper the bottom side smaller than top side about 0.07 mm.
So please you measuring actual diameter of drill cutter (position same as depth of drilling hole), and could you try for change the new drill cutter for compared the tapping result.