Blocks that emit light such as Shroomlights, Glowstone, and Sea Lanterns do not display the same light texture on all sides of the block. In all cases, the top of the block will emit a brighter light than any other side of the block.

This is easier to see when surrounded by blocks on all sides, the attached example displays the top of the block on the left and the side of the block on the right, both were screenshotted directly from the 1.18.2 latest release.