The slime/honey blocks does not show shulker boxes behind them, except when looking at the block. The pictures below show this better than I can explain.
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Fixed in 1.15 pre-1.

I'm unable to reproduce this in the affected version, so I cannot confirm if this is fixed or not in 1.15 pre-1. Someone who clearly could reproduce this bug before please tell if this is fixed or not.
Confirmed fixed in 1.15 pre-1
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I'm unable to reproduce. Might be dependant on the graphics card.