The biggest part then might be that they should have listened to the operators warnings and scheduled a proper re-inspection of the route once they started warning of issues.
Defects in laying the tracks have a chance to show up on an inspection, either the final one when building or one done at the regular intervals. If it doesn't shows up, your inspection is bad. If you can't inspect what you build, you can't build it.
Also modern high speed rails are built with continious welding without thermal expansion joints.
https://www.youtube.com/watch?v=zqmOSMAtadc goes through track building and the effects and literally mentions at about 12:30 that they do need to do inspections when it gets cold if the track cracks (there was a photo linked in another post about a cracked track).
Is it the cause? No idea but doesn't feel far fetched.
Edit: Seems official investigators are now even pointing to that as an initial theory, https://www.reuters.com/world/europe/investigators-find-brok...