Not going to be claiming to be an expert, but buckling is a well documented phenomenon and I'd be surprised if there wasn't possible issues due to contraction on the other end of the spectrum when it gets colder, the track was laid this last spring/summer so it's probably not been as cold for long before on the tracks.

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...