In 1.16.1 respawn anchors would respawn you in the 1x3x3 area above them, in the eight blocks around them, or in the 3x3x3 area below them, and you'd only respawn at the world spawn point after selecting a respawn anchor as your respawn point if it ran out of glowstone or if all of the above areas were blocked.

Since the 1.16.2 update, respawn anchors will only respawn you in the 1x3x3 area above them or in the eight blocks around them. If those are blocked it will send you to the world spawn point even if the area below them is completely free and available.

This completely disables putting respawn anchors in the ceilings of builds, which is pretty annoying.