Not weird at all. You'll see more welds like that the further back in time you go with bikes. They just didn't worry about cleaning up welds as much as they do today.

Many bottom brackets were made from a flat piece of steel. Though it might have been sold to the bike builder as a round piece already.

Many will be surprised at how much tubing and piping is made from flat sheets welded up into a round and then the weld removed by grinding and other processes.