A few random related thoughts:

I don't know why the program sometimes does not play a certain chord, particularly an upper extension, but sometimes as simple as playing a major versus dominant seventh (horrors). My point is, I don't think it's because it cannot; I think it's because some algorithm chooses not to. And I'm not including the bass in this comment; Dzjang did excellent research on the limitations there.

If you examine what BIAB exports to Music XML, you learn a few more things. For example, you mentioned 7#11. A 7#11 and 9#11 are both exported as 9#11. For decades, 7#11 was not supported; you could not even enter it, despite its popularity in jazz fakebook charts. Now you can enter it, but it appears to be playing as 9#11.

NC is listed in pgshortc.txt but has never been supported as a keyed abbreviation for No Chord. Instead we use something like C. which works fine but messes up the printed leadsheet.

And then there is the 2 chord. A 2, add2 and add9 are all equated to a 2 chord in BIAB. Look at pgshortc.txt to see this.

In the case of Natural Arrangement, changing what I wrote makes some sense, although we have no idea what that algorithm is.

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.