I’m glad I ordered so many spares now!
I think I now know the problem; my super-strong, super-light, super-balanced props aren’t so strong when they hit the ground vertically. Fly Phoebe into the shrubbery and she makes a great hedge trimmer. Crash her in a way that the prop hits something head on (rather than side-on), and it imperceptibly weakens the blade.
Next flight on take-off, the weakened blade can’t produce lift. so Phoebe flips over it. It breaks, and its diagonally opposite partner gets weakened in the process, but slightly less.
Next flight on takeoff, Phoebe actually makes it into the air, but that stresses out the weakened prop so she flips again, and both the weakened blade and its diagonally opposite partner get broken.
£38 worth of props bite the dust, metaphorically and literally.
And my motivation for further flights today is significantly deflated.