I have not found reproduction steps for this yet, but it only started happening after I upgraded to 1.3 rc4 from the previous 1.3 beta release. I’ve had a few incidents of hanging since upgrading.
The most recent time, it happened when clicking the ‘home’ tab. Aseprite locked up for 10 seconds or so, and then when it became responsive again, the brush size control disappeared from the top toolbar and the brush tool no longer painted, requiring me to restart the program.