To reproduce, place a bed so that the pillow section is outside the world border, click on it to see the message in chat that says "Respawn point set", and then die to respawn outside the world border. You can also sleep in it to go outside the world border. Expected is that spawn points are obstructed whenever they're outside the world border.
Linked issues
is duplicated by 2
relates to 4
MC-183899
You can set your spawn point inside an end portal, causing the player to become stuck in the End
Resolved
MC-153658
Villagers pathfind to and interact with beds, workstations, and bells outside the world border
Open
MC-187495
Spawn points are still valid underwater
Open
MC-117800
Half bed can be placed outside the worldborder
Resolved
gaspoweredpick
Another thing I have discovered is that no matter how high you set the spawnRadius, you never spawn outside the world border (as expected), so it is also expected that you shouldn't be able to have a spawnpoint outside the border.