[MC-251034] Multiple-blocks-in-one-space blocks have inconsistent placement behaviour

The bug

There exist several blocks in Minecraft in which several instances of them can be placed within a single block space. For this report, I will divide them into three main categories:

Stackables: all slabs, snow layers
Groupables: all candles, turtle eggs, sea pickles
Coverings: vines, glow lichen, sculk veins

In certain situations, inconsistent and undesirable behaviour can be sighted when placing these blocks.

Placement	Stackables	Groupables	Coverings	Notes
On top of a base block	Generally impossible	Stack increases	Generally impossible	Stackables and coverings cover the full face of the target block.
On the side face of another block	Stack increases	Stack increases	Generally impossible	Coverings cover the full face of a target block.
On the top of an existing instance	Stack increases	Stack increases	Amount increases
On the side of an existing instance	Generally impossible	Stack increases	Group increases	Stackables, being square-based and 16x16, do not permit non-flush placement.
On the side of an existing instance, flush with wall	New stack created on adjacent block if possible	Generally impossible	Stack increases	No groupables fill out a full block.

Expected results

For vines, glow lichen and sculk veins, targeted sections would create a new block on any adjacent walls.

Actual results

Doing this causes a new instance of the vine/sculk vein/glow lichen to be created inside of the targeted block. If no space exists for one, placement fails completely, even if there is valid space for the vine/etc. to be placed on in the adjacent block.