Summary:
Before 20w45a washing a banner in cauldron did not reduce the water level in creative mode, which was intentional per MC-63058. Now it reduces the water level even in creative mode.

Steps to reproduce:

1. Change your gamemode to Creative.

   /gamemode creative
   

2. Place a cauldron with water.

   /setblock ~1 ~ ~ water_cauldron[level=3]
   

3. Right click the cauldron with banner.

   /give @p minecraft:white_banner[banner_patterns=[{"pattern":"minecraft:cross","color":"pink"}]]
   

   → Water level of the cauldron was lowered