Instances seem to have detached themselves from master components?

I spent a couple days last month tiding up the components page for a project which is finishing. I made sure every master component and instances were all correctly linked and any old components not used were deleted.

I’ve come back to the file after not touching it since and some of the instances (all are text ones) are asking me to restore the master components. The components which they were linked to are still there but for some reason they are no longer attached. Anyone encountered this or know why they might have detached?