What I did: I inserted a fake newline by adding a :before-string “\n” overlay in the middle of a link. 
The problem: the underline goes all the way to the right (without :extend t)
Reproduce: 
1. Open an Org document, insert a link (C-c C-l some text RET some text RET)
2. M-: and run this code:

(let* ((beg (point))
       (end (1+ (point)))
       (ov (make-overlay beg end nil t)))
  (overlay-put ov 'before-string "\n"))

What I get (overlay line break):


What I want (insert a hard line break):


In GNU Emacs 27.0.60 (build 3, x86_64-apple-darwin19.4.0, NS appkit-1894.40 Version 10.15.4 (Build 19E287))
of 2020-05-16 built on missSilver
Repository revision: f27187f963e9e36435b508e29256e048799e0ff2
Repository branch: emacs-27
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.4

Recent messages:
Quit
Making completion list...
Quit
flywrap-insert-newline
Entering debugger...
Back to top level
next-line: End of buffer [2 times]
Auto-saving...
Quit
s-b is undefined

Configured using:
'configure --with-modules --with-pdumper=yes
--oldincludedir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/libxml2/
PKG_CONFIG_PATH=/usr/local/Cellar/zlib/1.2.8/lib/pkgconfig:/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig'

Configured features:
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NS MODULES THREADS PDUMPER LCMS2

Important settings:
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  value of $LANG: en_CN.UTF-8
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Major mode: Org

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  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
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  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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Memory information:
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