The Respawn anchor, if placed on the side of the nether worldborder, and surrounded by blocks, will cause the player to respawn behind the world Border.

What I expected to happen was...:
The player should be spawned on spawn, because the respawn anchor does'nt have any valid blocks to let him spawn.

What actually happened was...:
The player spawns behind the world Border

Steps to Reproduce:
1. Place an respawn anchor on the world border in the nether
2. Surround it on all sides with solid blocks, except of course the side of the border
3. Set the anchor as your respawn point

4. /kill

5. You spawn behind the border

Sorry for bad English