Drive flange fracture

[POD]

Wondering if anyone else has experienced this failure mode in a drive flange, or knows what may have caused it? It was an incidental finding when I dismantled to service wheel bearings on the 130. Wasn't even leaking. Has not done much work, I replaced rear shafts and flanges a couple of years ago so had a set with worn splines to temporarily install. I've contacted Ashcrofts a couple of days ago to see if they can suggest a cause but no response yet.
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[JDNSW]

That has a screw on steel cap, hasn't it? My guess is it hit a rock or something similar pretty hard. The alternative is faulty heat treatment, although I suspect it is meant to be pretty hard to prevent spline wear.

[p38arover]

Very similar in design to the MaxiDrive HD drive flange MD839X

Rear Heavy Duty Drive Flange Extended Spline - MDE859X (https://klr.com.au/store/land-rover/defender/axles-suspension/rear-axle/drive-shafts/rear-heavy-duty-drive-flange-extended-spline/)

Or the FRC5806A HD drive flange

Heavy Duty Capped Drive Flange - FRC5806A (https://klr.com.au/store/land-rover/perentie-and-110/axles-suspension/front-axle/drive-shafts-cvs/heavy-duty-capped-drive-flange/)