A question on a combined use of “$x Design and construction” and “Law and legislation.”

I have seen patterns like this:

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We know that “Law and legislation” can be subdivided geographically; but “Design and construction” cannot (at least questionable, since in the authority record, what we see is[log in to unmask]">). OCLC Quality Control seems to be in full control of that--no immediate geo. subd. after “Bridges—Design and construction.”

However, once there is an intermediate topical subdivision, things get a bit more complicated (as shown in the above two examples). The two headings appear to be syntactically sound. But the “FAST Convert” program detects an error. Why? Is it because the following heading should be used?

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650 #0 Bridges $x Law and legislation $z Kentucky. (convertible in FAST)

Is that it? What’s going on?

BTW, just out of curiosity, I have also tried both “Bridges $x Design and construction $z Kentucky” and “Bridges $z Kentucky $x Design and construction.” “FAST Converter” accepts both as “correct” and is able to convert.