Bug canvas rect wrong when camera set to RenderTexture, 2019 break?

Trying to fix a UI issue that was OK in 2018 but in 2019 LTS seems to be an issue.
If you do camera.targetTexture = someRenderTexture;
then the camera will immediately have a new pixel width/height based on the render texture, but if I look at a UI canvas assigned to that camera if will currently have a canvas rect based on the screen size & aspect ratio (screen space canvas, scale with screen).
The UI does not seem to work properly here as it is wrongly scaled when drawn in to render texture - appears to use render texture resolution not virtual resolution from canvas scalar

How can I immediately update that canvas rect & scalar to fix?

TLDR if want to change camera settings and render UI for 1 frame then canvasRect.rect for UI canvas will be wrong in the first frame (updated at frame end?) and you may have difficulty placing things over say world positions (non-screen-aligned positions)