https://gitlab.synchro.net/main/sbbs/-/merge_requests/440#note_5382

All ASCII characters (and thus, ASCII strings) are also valid UTF-8 characters/strings.

So... I'm not clear on what you mean by "sometimes"? If there's, say, a CP437 character in a string, then  *that* string would not be a valid UTF-8 string, but all purely ASCII strings are valid UTF-8 strings.

The JSDOC for this function says "Return true if the specified string contains only valid UTF-8 encoded and US-ASCII characters" - so I'm not clear if/how this function isn't behaving as expected or you just didn't read the "US-ASCII" part?
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