Downscaling with alpha = bumpy mess

Issue: If you use Image Size to reduce an image, the resampling breaks if the image has alpha. If it doesn't have alpha or if you use the Transformation Controls instead of the Image Size popup, it resamples properly.

Photoshop version: 25.1.0 (and earlier)
OS: Windows 10

Steps to reproduce:

1. Create a 512 x 512px canvas
2. Add a 450 x 450px circle and rasterise it if it's not already pixels
3. Go to Image > Image Size... and scale down to 64 x 64 (any resampling method)

Expected result: The circle should be smooth, as shown in the Image Size popup's preview
Actual result: The circle has bumps on top, bottom, left and right edges

Additionally, if you merge the layers before you resize, or use the Transformation Controls (Ctrl-T) to resize the circle, it comes out smooth when resized. The relative size of the circle and canvas affects how bumpy the resized result comes out - 512 and 450 were chosen because they show the effect clearly.