Using the paint bucket tool changes layer thumbnail preview into a square

Hello!

I'm drawing some pixel art.

Whenever I fill in a area in the canvas with the bucket tool the layer thumbnail preview turns into a weird square.

Here's a screenshot:

As you can see, on the canvas I have that little monster thing and on the layer thumbnail it's a square.

Here are the steps:

1. I drew the outline

2. Painted with the bucket tool to fill in the shape

3. Layer thumbnail turns into a square shape.

I have tried making the thumbnail bigger but nothing changes.

How can I fix this please?
Canvas size: 50x50px

I'm using Photoshop (Beta) 24.7.0 20230628.m.2223 0770f83 x64 on Windows 11