If you press “enter” in the middle of a block, the block is split at this point and you get a new block on the same level with the text after the cursor.
If you now press “backspace”, the split is undone and the text is rejoined.
So far, so good.
BUT if the situation described above is a TODO or a block before a TODO, “backspace” will not rejoin the text but delete it.
The following picture shows first how “backspace” rejoins the two separated blocks and then how “backspace” deletes the back part.