I hope to make use of this on iOS for YouTube. Will be nice having that the YouTube site in it's own separate "app" instead of having to keep closing tabs.
I have this issue as well, but on iOS as a normal Safari tab. I thought it was some login token invalidation because of IP change (due to mobile connection) but a couple days ago it happened to me while I was in the same network so I'm not sure anymore.
Yeah, I've certainly only had it happen in Lemmy so far. It can be pretty bad, had to log in multiple times within minutes while I was actively browsing at one point. If I catch it happen on my Mac I might put in the effort to report a bug, if I can figure out a way to debug it since it's seemingly random.
It stands for progressive web app. For websites that support it you can create an app out of the website by going to share and add to dock. Gmail, WhatsApp, and Lemmy are good examples of websites that support it.