I was in the nether, mining quartz. while crouching on top of soul sand, I went near some lava which was at the same level as the soul sand. it was quite obvious that I never actually touched the lava, as it was abt ~half level because it was flowing. I must also mention that the lava was below me. I believe that this problem is probably caused by how the game checks if you are in lava.
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duplicates 1
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Thank you for your report!
However, this issue is a Duplicate of MC-150455.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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UPDATE: turns out this is true for any non-full height block. it happens even if the surface of the lava is below the top of the blocks collision