Off and on over many years, I've seen posts supposing that bikes held in trainers experience loads and stress differently than when on their wheels and free to wobble. Maybe you can just be one anecdotal statistic more. Maybe it's been put to rest and I shouldn't have brought it up.

On the other side, did you ensure the spacers or anything to keep your correct dropout spacing was used? If you were snugging a 135 drop down to a 130 or even 126 mm spacing, that might be an issue.

I too have had simple cracks in finish that weren't in the frame or actual structural part.