I don’t use PWA, I don’t know what it is, exactly.
My bug report is for the regular use of (meta) Discourse website.
The “Open all external links in a new tab” option is there to make sure that all pages that are outside of (meta) Discourse navigation (menus), will open in a new tab.
Leaving the original tab with the (meta) Discourse page you were at.
Here, after I click this image link, when I hit the “Back” button, I will have to wait that Discourse website reloads entirely (and cross fingers so that it ends up on the same previous post, not the case here):
It should have opened in a new tab, like this other random external link.