Agent_H,i find it curious that practically everyone else's axe is cracked at the front or side of eye, where the thinnest part of a head is.
The poll of your C. does not appear too beat-up either...It's possible that a fairly major mushrooming was cut off,but distortion of any critical degree is not really apparent(in photos at least).
I've seen a couple axes that were literally beaten to death,and they both failed at the thinnest section of the cheeks,never through the center of poll,and not without any visible distortion of those thinner sections either...
All this plus the looks of it,plus a gut feeling,make me want to say that that crack happened when that head was forged...
As a form of brashness,excessively sized grain,or simply working it too cold:
"
Definition - What does Cold Shortness mean?
Cold shortness occurs when a material is lowered to a temperature where it becomes brittle. A material can be ductile at one temperature and very hard and breakable at another if it is affected by cold shortness. Not all materials are affected by cold shortness. For materials that are affected by cold shortness, the temperature at which brittleness occurs varies from material to material...."
Again,a gut feeling more than any solid knowledge,but that crack to me looks more like a result of forging rather than abuse in later life of tool.
It's a possibility that this tool has actually hung together in this wise since it was forged...That crack begins and is wider on the inside of eye and was overlooked or neglected;on the outside was initially filled with paint then dirt/rust,or was not visible and became so only with later clean-up Or propagated,however slowly...
(all my jabber above is a waste of time as it still needs fixing
...I'm not a welder and could not do this without inclusions myself.But it wouldn't be too terrible of a challenge to a pro;i'd make a cut with an abrasive disc clear through the poll and do so in a V-config.;it'd save on much tedious grinding and should only take a few minutes).