When you have trapdoors next to a fence and go in the crawling state, closing the trapdoor above you and where you are standing on, will allow you to crawl trough a fence.
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Duplicate of [MC-193086]
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We're tracking this issue as MC-193086, so this ticket is being resolved and linked as a duplicate.
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This happens because a fence doesn't connect to an open trapdoor, so when you open it, you can approach the hitbox of the fence, and then, when you close the trapdoor, the fence changes his hitbox to a "connected fence" and, because you are inside his hitbox, you can clip through it.
I personally wouldn't count this as a bug