Pretty much as in summary.  I had a cake sitting on a crafting bench in my train station.  The station is hung with banners (mostly pillager if that matters).  I crafted a bunch of the new candles and colored 2 of them purple.  When I placed one on the cake I could see the banners through the cake.  That cake is maybe 2 years old so I thought it might be 'stale'.  Destroyed the cake and crafted a fresh one.  Same results.  I'm working on screenshots but onedrive is being a pain.

How to reproduce:

1. Place some banners
2. Make a cake
3. put a candle on the cake
4. manuever around and peer at banners through cake