OK, after restarting I was able to make a little progress and better determine some of the mechanics. Seems like I got stuck because of a level design error or inconsistency in the movement code rather than a bug with a mechanic.
However, I'm now stuck in -1:B and frustrated because I can't tell what happens when I try to make the last jump in the middle, whether it's actually impossible due to movement issues or whether I just need to adjust something.
I'm playing in Firefox Linux (snap) on a laptop which runs Kubuntu 22.04.1 with a Core i5 CPU and integrated graphics. Just in case that's helpful for any reason regarding my possible inability to jump high enough.