I have mostly seen this with Google and Instagram. Whenever I go to see the what's new section in the Play Store version update, I see nothing. For some apps, most of the times it just "bug fixes and stability updates", "made the app look better", etc. Back in the days, we used to get some details about the update, as in what changed. Google Maps introduces breaking changes and yet the changelog message remains the same.
> I have mostly seen this with Google and Instagram.
Its because you are actually the product. They have no incentive to provide you with more information. They just want your eyeballs and keystrokes so they can sell your attention and interest to advertisers.
I publish lots of apps and don't have a great explanation either. I guess if you say that you just fixed some bugs, neither the app reviewers nor the users will pay too much attention and just approve/download the update, which is what you want as the app developer. Or it's just laziness. Or a communication disconnect between the person in charge of writing the changelog and the product/development team.