Sheet Sets External Reference file has changed

Hi All. I have a little annoying problem that has popped up ever since I have started using sheet sets. The little bubble will pop up that tells me that one of my external reference files has changed. I will click to reload it and then a little while later it will pop up again. I know that it hasn't changed because I am the only one working on this project.

I have to open the offending drawing and save it to get the bubble to stop bugging me. Anybody know of another way to get this to stop happening in the first place?

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Sorry, but I don't have an answer for you, although I have experienced the same thing from time to time on drawings that I've inherited from other people. I'm not sure what causes this or how to fix it, so I would also be interested if anyone has a light they could shine on this issue.