Phew, the ‘New’ App Center Icon in Ubuntu is a Bug
At first I assumed its appearance wasn’t intentional but a snafu or screw up caused by me (I was playing around with display scaling options when the icon in the dock first changed).
But it wasn’t related to me truing out different fractional scaling options.
As App Center is a snap (snap-store ) it gets updated in automatically in the background. An update rolled out that included a new icon, which you can see below. It still uses the shopping bag motif but is 2D, one colour, and has transparent elements, more akin to a symbolic icon than a proper one...