That is the special palette animation. it cycles specific palette indices. The problem people tend to have is that when they want to replace the image, they will take the image they want, which is usually at a higher bit-depth, and convert it to indexed color to make it a 8bpp bitmap as needed. The issue is that the animation is done by palette cycling and the colors to cycle are specified in the bitmap header (very much not a standard use of said header), and of course the bitmaps people create this way won't have that, so things get weird when Windows tries to do the palette cycle animation.

I found this guy who made an alternative splash screen that mimics how some games loaded on the Spectrum, with flashing lines.

here is another example of somebody coming up with a replacement boot logo. They also describe the process better than I did. They link to a tutorial as well which has a neat little helper program that can add an animated bar to any logo image.