I noticed this happening too. I can add that it happens when you set the radius to other numbers as well. For example, if I set it to 100,000, I spawned at a range between Z: -90,000 and -99,999. When setting it to 500,000, I spawned at -499,000 or higher. It appears that the higher the spawn radius, the closer you are to the edge of the radius. At 30 million, I mostly spawn around -29,999,900, within 100 blocks of the border.
Can confirm it happened to me too. I updated from Paper 1.16.5 to Spigot 1.17. I thought it might be a Spigot issue, but the other commenters did it on vanilla.
Steffen Eide
I noticed this happening too. I can add that it happens when you set the radius to other numbers as well. For example, if I set it to 100,000, I spawned at a range between Z: -90,000 and -99,999. When setting it to 500,000, I spawned at -499,000 or higher. It appears that the higher the spawn radius, the closer you are to the edge of the radius. At 30 million, I mostly spawn around -29,999,900, within 100 blocks of the border.