One would expect that either only the section punched would crack, or both would. However, this varies from block to block, rather than being standard.
Bed: only the targeted half cracks, although both break
Door: only the targeted half cracks, although both break
Piston: only the targeted half cracks, although both break
Chest: both halves crack, but only the targeted one breaks
I can't test double plants since they break instantly.
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In 19w44a, this seems fixed, as in all cases only the targeted half cracks.
Hard to test in 41a.