I've followed roller coasters for longer than I want to admit, and know of very few instances of something like that. Not on modern, computer designed coasters. It most certainly has to be a perfect weld, putting the new piece of track in the exact location in 3-D space so that it doesn't make things worse.
It may have been welded for some connection repairs, or hole repairs. I find it hard to believe it was a nightly occurance, as that would point to a pretty massive safety issue developing, and the frequent welding logs would be all the proof needed for an injury lawsuit. But I've never heard of any park replacing feet long pieces of track, by a couple welders at night. Because, you know, they didn't just bend a pipe in the shop and weld it in.