Banners are visible through blocks like honey, slime or water as though you're looking through glass.

Banners are visible through blocks like the honey and slime blocks or water.
In order to replicate: Put a banner on land, close to water. Hop into the water. The banner is visible as though you have X-Ray. Alternatively, place a banner on land, place two honey blocks in front of it and stand behind the honey blocks. Same effect.

Attachment below if I didn't explain it well enough. (the player on the screenshot is in water.)