In at least version 1.16.5 and in all of the snapshots, if you activate crawling mode and hit the jump key on your keyboard, you will bounce slightly upwards (and of course hit your head on the block above you). I would assume this is a bug, as it doesn't make any sense logically why you would be able to do this, but it may be intended.
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It's not a glitch. There's a bug report saying the opposite and is marked as resolved.
Player cannot jump while crawling
I would assume it is intended, but it is confirmed in 21w06a.