Okay, I pushed a fix for this to PageEdit master that will now make it handle any and all external links (any non-relative target - ie. has ':' ) as special cases and therefore not try to build a relative link from the source.
Tested it with
https://www.mobileread.com/forums and once PageEdit was in preview mode, the link worked just fine. Triple checked with the Inspector and it should now work.
I still have to screen out #PageEdit_Injected from the SelectHyperlink dialog building.