When crawling in a 1 block tall opening and mining below you, you can only reach 4 blocks. When trying to mine a fifth, it looks like you can reach it, and it seems to break, but the block reappears.
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This appears to be a duplicate of MC-183309.
Dupe of MC-183309
Seems like the server treats mining range from player's feet, and offsets it, rather than working from the eyes, so the server thinks it's out of range.