When you are holding a brush and you tap the background of the inventory, the brush gets used.
How to reproduce:
Place a suspicious sand block down.
Open your inventory
Get a brush
Hold the brush
Open your inventory again
Tap the background of the inventory [Video|[2023052521551400_s[1].mp4|https://bushy70-my.sharepoint.com/:v:/g/personal/fox_bushy70_onmicrosoft_com/EXg7X2i5SdhHvwht02uq0H0Bj-UaujWXUNDU94tG8bT6Jw?e=OobFSF]]
(This bug has been resolved as of 7/13/24)
It's up to Mojang to decide if it's WAI or not. You don't need to decide it; Since your report has ADO, they consider this is a bug for now