Yep, and it's really frustrating when the tab was just a static webpage with some text you want to read. It has the iOS baked-in assumption that you always always have fast Internet access.
Putting something into your Safari Reading List saves it for offline access. If you want to read a webpage later on mobile (and it won't parse through Instapaper/Pocket for some reason), that's probably the way to go, vs. leaving it open as a tab.
Though, admittedly, I never got it to work right at actually using the offline-snapshot versions of pages when I was, er, offline. Even though it clearly had them taking up space on the disk.
Shouldn't this be handled by website developers via PWA-related tools? Think about it, if you're website is meant to work offline (and safari cannot know this), you can implement the necessary code for it to work offline (e.g. web.whatsapp.com)
Expecting the page to stay open until the tab is closed isn't normally considered to be "working offline". The main point of a PWA is being able to navigate offline, not just continue to exist.
For me it's often something like a manpage I have had to find via Google. The web developer isn't thinking about stuff like that because it's a pure Web 1.0 static page that normally doesn't need anything like that.
Though, admittedly, I never got it to work right at actually using the offline-snapshot versions of pages when I was, er, offline. Even though it clearly had them taking up space on the disk.