Music frames get labeled according to two parts. The prefix “M” means it’s a music frame created on a master page. If it had been drawn directly onto the layout (thus creating a page override), it would have had the prefix “L.”
Then the second part labels frames in the order they were created. MA, MB, MC…MY, MZ, MAA. But it’s important to know that text frames are also part of this! So you have a music frame on the page, MA, and then if you have three text frames on a page, they will be -B, -C, and -D… and if you draw a new Music Frame, it will then be ME. That seems confusing because you can’t see the text frame labels, but they are there! I assume in the future Dorico will add the ability to chain text frames together, just like music frames are now.
That unique letter designation (MA, for example) indicates a Music frame chain. All music frames with that label will chain together. That’s how you would make a piano duet score. All the left pages would use one Music Frame chain (MA, for example), and all the right pages would use a different music frame chain (MG, for example). So the music would flow from MA to MA frames, and from MG to MG frames.
Your colleague must have created lots of text frames maybe. But it’s not too difficult to fix, as you’ve found. As long as the label is correct for the music frame chain.