Summary:
If you have your respawn point set using a bed or respawn anchor, the game avoids respawning you on dangerous blocks. If you don't have a spawnpoint and therefore respawn at the world spawnpoint, this is not the case. If the gamerule spawnRadius is higher than 0, the game could avoid respawning you at dangerous blocks. This issue also allows you to make spawning traps for new players.
This affects the following blocks (list may be incomplete):

• Magma blocks
• Powder snow
• Wither rose
• Fire
• Soul fire
• Sweet berry bush

This does not affect the following blocks:

• Campfire
• Soul campfire
• Pointed dripstone
• Lava cauldron

This is likely due to them being non-full blocks.

Steps to reproduce:

1. Create a superflat world

2. /gamerule doImmediateRespawn true

3. /fill ~5 ~-1 ~5 ~-5 ~-1 ~-5 minecraft:magma_block

4. Run the following command multiple times:

   /kill

Observed results:
Sometimes you will respawn on a magma block.

Expected results:
The game should avoid respawning you on dangerous blocks like magma blocks in this case.