Can you please provide a print illustration? Since this is a sign of operation, it would normally occur between two numbers and the numberic indicator would have occurred before the first number. Section 9.(8) on page 6 of An Introduction to Braille Mathematics provides an example.
Yes, Section 134 of the Nemeth Code says that when these are arranged either vertically or horizontally, the combination of them must be regarded as a single sign of operation. It also says that the numeric indicator must be used before a numeral following a minus sign when the minus sign follows a space. In this case the number doesn't immediately follow a minus sign (it follows a "minus or plus sign") and the number is separated from the minus by a plus. My personal opinion would be that a numeric indiator is not necessary, however I do not have a specific rule to back my opinion. If you can provide a print example, I would be happy to pole the rest of the mathematics committee members. We provide this service to assist in your braille transcription, not just to address "what if's" or to solve disagreements. 🙂