Replacing spec of display text property uses wrong when set as before-string or after-string property of an overlay

[Akib Azmain Turja]

Here is an example code:

------------
(let ((txt-disp (concat " " (propertize "_" 'display "foobar") " "))
      (ov (make-overlay (point-min) (point-max))))
  (overlay-put ov 'before-string
               (propertize txt-disp 'face
                           `(:weight bold :foreground "red"
                                     :background "yellow"))))

;; To remove the overlay call `delete-all-overlays`.
------------

After executing the above " foobar " should appear at the beginning of
buffer with red foreground and yellow background.  But the "foobar" part
doesn't use any color (i.e uses the face of the real text at the
beginning of buffer).  This doesn't happen when the text is inserted.
Is it a bug?

-- 
Akib Azmain Turja

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