SemBr author here. I chose SHOULD NOT for this rule for a couple reasons: First, as an affordance for text with ambiguous or unknown meaning. Second, as a hedge against introducing new meaning unintentionally.

Adding a semantic line break inherently changes the relationship between words, and we can't always be sure about the intended meaning of text. If this were MUST NOT, then any modification would risk violating the spec.

Then again, this may be my own, idiosyncratic reading of RFC 2119. If you'd like to discuss this further, feel free to open an issue on the GitHub repo here: https://github.com/sembr/specification/issues