The bug
Look around in a Giant Tree Taiga or plant a giant spruce by bonemealing 4 spruce saplings together and you may get a very short and out of proportion giant spruce tree. This does not apply to giant spruce trees from Giant Spruce Taiga.
Linked issues
is duplicated by
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To find a "giant" spruce tree that is wider than it is tall, use the seed 8 and go to to -1021 72 6.
This can also be reproduced by growing giant spruce trees by placing 4 spruce saplings together and bonemealing them. The resulting tree may be too short, or even a mere bush. The screenshot shows a tree I grew myself in a giant spruce taiga buffet world (where the trees aren't glitched unlike in giant tree taiga).
Ah, thanks. Didn't realise it affected trees from saplings as well. Can confirm.

Confirmed in 19w44a.
What are the seed and coordinates?