I've found this to be an issue ever since I first started using indesign 3 years ago. Sometimes when shift selecting objects, other random objects that you have already selected will unselect themselves. For example, I am selecting 4 random objects on my spread, I select 1, then 2, then 3, then 4, and object 2 unselects itself when I selected object 4
[Deleted User] commented
Have been having the same issue for years.