Apple refuses to implement PWA features in Safari, and they don't allow you to run other web browsers unless they are skins over Safari's WebKit.
Safari on iOS deletes LocalStorage after 7 days of inactivity (https://webkit.org/blog/10218/full-third-party-cookie-blocki...)
> As mentioned, the seven-day cap on script-writable storage is gated on “after seven days of Safari use without user interaction on the site.” That is the case in Safari. Web applications added to the home screen are not part of Safari and thus have their own counter of days of use. Their days of use will match actual use of the web application which resets the timer. We do not expect the first-party in such a web application to have its website data deleted.
> If your web application does experience website data deletion, please let us know since we would consider it a serious bug. It is not the intention of Intelligent Tracking Prevention to delete website data for first parties in web applications.
The same sites can handle doing background media when used as a regular website in Safari, so this feels like one instance of PWA support at least seeming intentionally crippled.
The feature/bug argument doesn't really apply here - your preference is not the same as everyone else's, and your worst case scenario shouldn't prevent valid uses of the same technology. Especially when it's already something the user has chosen to install.