Bug: the vertical mirror builtins `∊` and `.∊` act weird in some cases when list arguments are given

[kcruijss]

I'm not sure how to describe it, but there are some weird and inconsistent things going on with the vertical mirror builtins:

1. ∊ with a list input: works as intended, and results in a list (as expected): try it online.
2. .∊ with a list input: works as intended, but implicitly joins by newlines afterwards (why not a list as well like with bullet point 1?): try it online.
3. Either of the two after we've prepended or appended to a list with ª/š: doesn't do anything: try it online 1 or try it online 2.

Bullet point 3 is primarily the bug here, although I also would have expected bullet point 2 to result in a list similar as 1.

EDIT: Apparently it's not just after an append or prepend. It happens after almost any list manipulation. For example after a flatten ˜ as well: try it online.

The builtins work fine on strings, so a temporarily work-around is joining and splitting by newlines before/after the vertical mirror: »∊¶¡.

Found it while golfing this codegolf answer of ovs.