Confirmed, as I can reproduce the bug by following the steps that Icho details in the bug report.
The important thing is that there must be no frame advancement when the level loads in combination with pressing the pause hotkey while on the black screen. If you press pause while the screen is black, unpause after the level has loaded to allow some frame advancement, and then quickly pause the level again, then finally load the replay, there is no problem at all.
The level I noticed this happening has no entrance, but I have confirmed the bug happens in levels with entrances as well. Also, the bug happens with Icho's level in my video, but I have confirmed that it happens in the other R2 contest levels as well, and so the problem is not tied to the tileset. Finally, I have checked that this happens in v12.10.3, so this bug has been happening for several NL versions. Luckily, no replay breakages, but still something to report. Thanks, Icho, for doing so