I think this might be the problem. Looking through the forum it seems like this isn’t the case; apparently, golangs filepath package doesn’t support **.
The restic docs mention that files-from specifically uses filepath.Glob, and the docs for that seem to confirm it doesn’t support ** (there’s also an open issue, requesting the addition of support for **).
As an odd aside, the example exclude syntax in the restic docs specifically uses **. I’m not sure why that is, when I tested the behaviour, it behaved as two *'s, rather than a single **.
Anyway, if I understand what you’re after, essentially you want to back up only a specific filename in an arbitrary number of nested directories?
If so, I can’t think of a particularly “nice” way to do this in restic. The only suggestion that comes to mind would be to pass restic a list of the files generated using find via the --files-from-verbatim option, generated before each backup. The restic docs have an example showing this.
Sorry that’s probably not exactly what you were after, hopefully someone else has an idea for a cleaner solution.