Any reason why this should happen?

I created my index page, and duplicated that so the header, footer etc would be the same on all my pages. So I added all the columns, images etc to all my pages and exported it. Then I created my menu with MB and went to paste the html code using the HTML editors find & replace function.

I highlighted this bit of code <nav class="coffee-span-9"></nav> and pasted the MB code into it. It worked for 19 out if 24 pages. When I looked at the other 5 pages they had this code <nav class="coffee-span-9 coffee-527-span-9"></nav> or some other variation of it instead of what was highlighted on my index page.

So what's happened? Have I unintentionally altered something in RLMP without knowing about it? I know it might not sound a big thing, but I'm now conscious that some of the pages might have random bits of code in that make it difficult to bulk change bits in the HTML editor....

Are the six pages with the issue definitely ones that were just duplicated? Did you go back and look at those pages in RLM and see if they have something different about them?

It's either an original duplicate, or a duplicate of the duplicate if that makes sense? There isn't anything different that I can see in rlmp?

Try duplicating more pages in the project and see if they do the same thing.

They don't seem too. Just happens to be these five random bits from what I can see at the minute.

If I had to guess it is because you didn't highlight the entire code but only the class name maybe? Seeing as the other code that you showed us does "include" the class that was part of that code, it would replace it anywhere it found that class including places that had other classes in them with it. The search function is exact so it can only search for what you ask it to search for, it would be highly unlikely that it looked for anything that contained your search query unless you didn't include the entire code with brackets etc. check also to see if there are settings for it that might ignore the brackets, but I don't think there is.

CASE 1: <nav class="coffee-span-9"></nav>
CASE 2: <nav class="coffee-span-9 coffee-527-span-9"></nav>

In RLM exported code:
-- Case 1 above tells the browser that the column is 9 spans.
-- Case 2 above tells the browser that the column is 9 spans, but at the 527px breakpoint it is 9 spans.

All pages of your project must have the same set of breakpoints. However, while pages share breakpoints, the columns on those pages do not all have to have the same span count. Therefore, if you check the original exported files and find that the code is different (like Cases 1 and 2 above) for the 5 pages that did not update correctly, you must have made changes to the column span on those 5 pages in RLM before exporting.

At the breakpoint set at 527 pixels, at some point you changed the column span for that breakpoint in RLM and then perhaps changed it back for some reason. Therefore, it kept the column span for the smaller breakpoint.