I thought it looked like a bug, but it's a feature based on which level you are on and how many lines you've filled(?). Here's the part that decides if it should fill or not: https://github.com/raulgrell/tetris/blob/master/src/main.zig...

I've seen that behaviour in other Tetris games as well. :)

I noticed this too, but it correlates when you go up a "level" - so I assume it's a feature to add random rows each level.

More than one person has mentioned this. Maybe an audio cue will make it more obvious it's on purpose...