Thanks for the tip. Unfortunately, other aspects of upgrading completely from jessie to testing/stretch made it not worthwhile vs. the justification (I wanted the `-J` switch available with newer versions of `lsblk`), so since there wasn't much to loose I've scratched the system and put it back to Raspbian 8.
You have given me something to think about and look into in terms of my packaging though. A point slightly contra this theory, though, is that it worked fine with 5.20.2, some machine, same distro. This doesn't mean it isn't a factor though, see the part of my second response to Anon monk #1 regarding reports of the same problem online clustered last year and all involving 5.22: Re^4: "loadable library" mismatch on new installation.