The plot thickens. I went back to an older sequence and discovered that it’s happening there, as well, and may have always been happening with my Octopus.
The reason I didn’t notice it previously is that a note on any page has the effect of my last sentence in the post above this one. That is, when my drum page is playing, they create the "in between" notes which cause the monophonic synth page to play properly.
The monophonic synth page is playing 4 notes, such that the first two notes are legato and the last two are stand-alone.
If the drum page is playing, with a note anywhere from the first beat (where the first monophonic note is on the other page) to before the second note of the monophonic page, the monophonic part plays properly. (That is, it plays: Cnoteon, D#noteon, D#noteoff, Cnoteoff).
If there are no notes on the drum part before the D#noteon, but there IS a note before the Cnoteoff, then I get the following: Cnoteon, D#noteoff, Cnoteoff.
If there are no notes on the drum part before the Cnoteoff, then I never get a Cnoteoff. I get Cnoteon, D#noteoff. If I get rid of the D# note, as well, then the Cnoteoff plays correctly, regardless of what’s going on in the drum track.
The drum notes are different than C and D#. This seems to happen regardless of page in the grid and what channel the monophonic part is playing on, and is independent of the device that it is playing. I may have never noticed this before because I usually start by programming a drum part.
Can someone try to recreate this? I’m 99% certain I’m on the latest non-beta OS.