Did some testing. The bug occurs when the window width is less than 832 (standard panel) / 640 (compact panel), and/or the window height is less than 400 (either panel). I suspect the issue arises from some code intended to prevent over-zooming the panel, incorrectly limiting it to 0 instead of 1 in this case.
EDIT: Found a more-direct cause of the bug - the bug occurs if the panel's zoom, upon starting the level, would be 1x. If there is no reason that it cannot increase above 1x, then zooming in will fix the issue (and it will remain fixed even if you zoom out again). This means, the bug can also be triggered by disabling the "increase zoom on small levels" option and setting your default zoom to 1x, regardless of screen / window size.