I have a case going with Apple, reported through the Feedback app. I've sent them a project that confirms the bug, a video showing the memory growth when using Xcode 13.1, and a link to this forum thread.
In my case, the memory running wild happens if the Canvas fails to load/build a preview. If I rebuild the project (Commad+B) after the canvas fails to load, the swift-frontend processes are killed.
I have stopped using the Canvas for now, and instead use Command+R to preview changes in the simulator, until Apple fix the issue.