Considering the case of app updates, why do app developers generalize changes in their update notes?
How does each writing style (general or specific) for update changelogs affect UX?
App updates are an important platform for communicating product changes and improvement. They are a practical place for communicating information, and therefore are the domain of UX writing. App update notes are not the domain of marketing and copywriting. This is not the place to sell to your customer how great the app is; it's the place to tell them what you did to the app and how that impacts them.
There's a couple of different reasons for generic app update notes:
There's great resources out there. This one is a decent one.
Re: styles (funny, serious, friendly, etc.), it's really the brand voice that determines how the App Updates are written. App updates shouldn't be a comedy routine--the user needs to know what's going on and what's changed--but if the voice of the company is light hearted and fun (like Slack), then their App Updates are going sound lighthearted and fun (while effectively communicating the updates).
I think it's often because of marketing and security. It's important to not reveal that some vulnerabilities to the general public, both so that people don't try to further exploit them or look for similar ones, and so that the perception is maintained that the product is solid and only has minor issues.
You'll find that the notes are more specific when discussing a new feature that will get users excited.
I think the funny/playful ones are not funny- if I wanted a laugh, I wouldn't be reading the update notes of an app.
I suspect apps like Facebook, which rarely if ever provide specific release notes, generalize due to the high number of different versions they may be testing at any given time. (https://motherboard.vice.com/en_us/article/8x8wp4/why-navigation-bar-looks-different-facebook)
For everyone else, most app updates just don't include information relevant to "the average user." Unless a bug was causing catastrophic issues, most users don't need to know which bugs were squashed in which update.
New features or big changes might be called out in the release notes but there are arguably more effective channels to communicate updates like that, such as in the app itself.