If you stand on piece of wire and jump up, you can't place wire under yourself. Same goes for sneaking to edge and trying to place an adjacent wire. This makes placing wire lengths very complicated and was not present in past versions.
[1.90] [Bug] Can't place wires while standing on them
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- Official Post
Was present in quite a few past versions.
Implemented around 1.2 (IC version, not MC) to fix the issue of people placing cables into their own hitbox.You can still place thicker cables below you while jumping, based on the fact you jump high enough to leave the blocks hitbox.