When windows = true, Fpath does this:

# let d, b = split_base (v "A:B:C") in to_string (d // b);;
- : string = "B:C"
# let d, b = split_base (v "\\\\server\\share\\foo:bar") in to_string (d // b);;
- : string = "foo:bar"

(I think colons are invalid in segments of Windows paths, so the issue might be that v allows such silly paths rather than that split_base and // give a strange result)