The bug

When you set your spawn to a bed or respawn anchor with a command, the respawn block behaves as if you used it, with the exception of you respawning there even after the respawn block is destroyed. However, charge a respawn anchor and then use /spawnpoint on it, and you will deplete the charge until it is gone, and then the respawn anchor will not let you spawn there but destroy the respawn anchor and you can spawn there just fine.

The bug is either that setting a spawnpoint at a respawn anchor with a command depletes the glowstone charge, or that setting a spawnpoint at a respawn anchor with a depleted charge does not allow you to spawn there.

I think that the bug is the fact that the glowstone charge is depleted when you use /spawnpoint on the respawn anchor because it makes sense that the respawn anchor with a depleted charge would not spawn you because the block would be considered obstructed.