I have no idea why Safari Technology Preview has not picked up your bookmarks. I just searched my system using FindAnyFile and I only have one set of bookmarks for Safari and there are no separate bookmarks for Safari Technology Preview
I've got ~/Library/Safari/Bookmarks.plist, and I had ~/Library/STP/Bookmarks.plist, and as I said, even after I trashed the latter, replaced it with the former, and did a cold start, STP still reported the same bookmarks as were in the trashed file.