The bug
The effect for entities swimming in lava ignores the height that the lava actually is. This means that if lava exists in the block space in any form, any entity that enters that block space will take fire damage (via end portal frames, jumping, etc.).
Any entity that enters that same space at the same height of a stonecutter will be capable of swimming upwards and take lava damage (via slabs, stairs, lanterns, stonecutter, jumping, etc.).
Linked issues
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Comments
Makzevu
Makzevu
In 1.16.0.59, lava no longer deals fire damage at full block height. Originally, fire damage would occur at any level in the block space; however, now it occurs .6 blocks high instead, the same level equal to taking lava damage. This means that the only way to reproduce this now is with any block lower than or equal to a stonecutter's height, since it's .56 blocks high.
Standing on an end portal frame, soul sand, or enchanting table will not ignite the player at all. The enchanting table method is interesting for the fact that the lava's model is visually above the block's height.
migrated
Affects: 1.16.100
Affects 1.16.200.56 Beta
Makzevu
In 1.16.0.64, the changes listed earlier are no longer valid. It's likely that this change happened in 1.16.0.63 when MCPE-74990 was fixed, but that beta also fixed a pathfinding issue with mobs on soul sand. This fix made soul sand the same size as a full block, therefore excluding it from the steps to reproduce this issue entirely.
Minecraft386882
Confirmed in 1.21.44
Relates to MCPE-46074.