Steps to reproduce:
The box-shadow is "cut-off" at the inline start or end side if the inline-box is broken into multiple line. The expected rendering is shown further down in the MDN article. Direct link: https://developer.mozilla.org/en-US/docs/Web/CSS/box-decoration-break/box-decoration-break-inline-slice.png
The box shadow is extended at the inline start or end side of the inline box. See the attached screenshot. Note: Enabling gfx.webrender.force-disabled yields the expected result.
FWIW, if taking the screenshot of the MDN page in comment 0 by right clicking and choosing "Take screenshot", the screenshot is actually shown the expected result.