There is no such thing as a default shape. You have to define the shape as something, Rectangle, Ellipse, Path, whatever.
If you change that meter to Meter=Image, then it works fine.
Or if you properly define the shape, then it works fine.
Shape=Rectangle 0,0,300,200 | StrokeWidth 0
You don't need the W / H / SolidColor on that. It does no harm, but it's redundant and pointless.
The reason that this appeared to work for you in an earlier beta was due to some "broken" mouse detection in earlier betas. It's almost too complicated to get into, but the long and the short of it is that your example is not "supposed" to work, and properly "doesn't". A Shape meter with no "shape" is invalid. It has no solid content in it that can detect the mouse. Mouse detection in Shape meters isn't like other meters.
It's not based on the meter rectangle created by W / H / SolidColor, but on the solid pixels of the Shape(s) created by the meter. Your example doesn't create any.
Mouse detection in a "content / container" relationship is based on both the content meter and the container meter. It must be detected in both.
Yes, it is a bit weird that the "content" meters display at all, in a sense they are being "fooled" by the W / H / SolidColor on the meter. In a perfect world they probably shouldn't even display in a Shape meter with no "shape". However, I'm not interested into digging into that, as I really don't care. A Shape meter with no "shape" is invalid, won't ever detect the mouse, and is of no value.
If you want to create a simple rectangular meter to act as a "container", either use Meter=Image / W / H / SolidColor, which is designed to actually create a real "image" based on those dimensions and color, or use a simple Shape meter with a properly defined Shape=Rectangle X,Y,W,H. I'm not sure there is any particular advantage to either choice in that case. I would probably use the Shape meter as I really like how Shape meters work, (and it's one option instead of three) but functionally it is six of one and a half-dozen of the other.
Yes, other meter types can be forced to play this role with W / H / SolidColor, as most (but not all) meter types will detect the mouse on any solid pixel in the meter, and as long as they don't actually spit out errors in the log if they have no functional value, they will create a "container" based on the W and H that can be used, but just DON'T. Use Meter=Image or a proper Meter=Shape for this.