Updated to 0.4.0 recently and came across a new bug.
Using the pencil tool, using the shift key to draw straight lines can lead to inconsistent results sometimes. The problem only seems to occur when drawing across multiple *mapped* tiles. I haven't experienced it on unmapped tiles.
In this GIF, you can see how it works. I draw a line in red while holding shift, then when I release the mouse to apply the line, the line changes shape.
I've been using Pyxel Edit for a year, and this bug is new in this version for sure.