Interesting observation, but no, it isn't a bug. Undo always undoes the last push not the last move. The reason for this is that moves which do not push are already intrinsically reversible with the same number of keystrokes (just press the opposite direction). On smaller levels this mechanic may seem unnecessary, but it makes undo a lot less tedious on large levels.
Viewing post in The Golem - Level of the Day January 1st 2019 comments
Ah, that makes sense, but I'm still seeing inconsistent behaviour in how that's applied, which stopped me from getting a feel for what it was doing.
Testing it outside of opening moves, if I push the first block down and walk around it to push the second block down, and then hit Z after one push of that second block, the game will sometimes rewind my last single push, and sometimes wind back two pushes (to where I'm about to push the first block a second time). If it's making a clever call about grouping less interesting pushes together and skipping them all, it's applying that inconsistently.