Since we check only the first element, a list such as [["my_string"], "another_string"] is considered valid, interpreting "another_string" as a list of 1-character strings. Is this intentional, or should we validate each element? If intentional, could we make this explicit in documentation

I think [["my_string"], "another_string"] is invalid. For me the question is if the validation should be exhaustive or "best effort" (this PR). It seems a likely cause of a user mistake is using a sequence of strings, instead of a sequence of sequences of strings. This means that checking the first item is enough to catch this common mistake.

I think I'd be ok with just checking the first instance. See if we get more bug reports and then act

I am open to deprecating as suggested in #13199 (comment), but from reading the original issue I think the current behavior is a bug.

I'm okay to consider it a bugfix

I think I'd be ok with just checking the first instance.

That's what we usually do in such cases.