Hey, @kidong_lee2392. Welcome to the Photoshop Community!😊 I'll help you figure this out.
What exact version of Photoshop, iPad OS & iPad model are you working on? Have you recently imported custom brushes?
Does this happen with a specific Brush preset?
What troubleshooting steps have you tried already? Another user shared in a thread from 2022 that reinstalling Photoshop helped them: https://adobe.ly/4iX4Iuy;
Please try reinstalling it and check how it goes.
Let me know if it helps.
Thanks!
Sameer K
(Type '@' and type my name to mention me when you reply)