`fold`ing Effectful `Path` crashes at runtime

This contrived code should check if a relative path is a directory. e.g. if I passed in ./repro ., there'd be no / and I'd just expect it to return Ok({}). Instead, it builds just fine but then crashes at runtime with: [ROC CRASHED] dispatch on a value that can never exist.

Full easy reproduction (AI genn'ed, but I tested) here

app [main!] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.22.0/F1JVZPYfWP71s8vk6tHcV1Qx1Ef6CZkwswGoCn8VHZmL.tar.zst",
}

import pf.OsStr
import pf.Path

require! = |root, relative| {
    path = relative.split_on("/").fold(root, Path.join)
    is_dir = Path.is_dir!(path)?
    if is_dir Ok({}) else Err(Missing(relative))
}

main! = |args|
    match args.drop_first(1).map(OsStr.display) {
        [text] => require!(Path.utf8(text), "")
        _ => Err(BadArgs)
    }

Start by running the linked BUG-006 reproduction with the Roc code shown in the issue, focusing on the effectful Path.join fold and the Path.is_dir! call. Trace the runtime dispatch that produces the crash. Done means the relative-path example completes without a runtime crash and returns the expected Ok({}) result.