This bug has occurred for as long as I can remember. When you use "/gamerule spawnRadius" to make a very big spawn radius, the player always spawns close to the edge of the radius. In the pictures below, I set the spawn radius to 30 million and I always spawned at the world border.

What I expected to happen was...:
I expected the player to spawn at a completely random point between -30,000,000 and +30,000,000 on both axes

What actually happened was...:
The player always spawned near the world border at around Z: -29,999,900.

Steps to Reproduce:
1. Set the gamerule spawnRadius to 30000000
2. Die without a bed or respawn anchor
3. Look at the coordinates

This is also the case with different numbers. If the spawn radius is 100,000, I always spawn around ~90,000 or closer. With a 500,000 radius, I would spawn at ~499,000 or closer. It seems that the higher the number, the closer you are to the edge.