So: jouncing over the larg(er) potholes / pavement discontinuities / debris on the roads around here wobbulates the front fender enough to pull the stays out of those tidy 18 mm = 6 diameter deep sockets on the fender clip.
Perhaps a generous application of heatshrink tubing will help:
Waving a heat gun around a 3D printed part seems fraught with peril, even with PETG’s glass transition temperature around 80 °C = 175 °F, as ordinary polyolefin tubing shrinks at 140-ish °C. Aiming the hot air stream more-or-less away from the clip (and the tire!) carried the day. PLA would surely have gotten bendy.
The proper solution surely involves screw clamps and suchlike. I really dislike fiddly hardware: I hope this hack survives.