Recipe:
start emacs -Q etc/HELLO
move point to character position 358 (ARABIC LETTER SEEN)
hit C-SPC to activate the mark

Expected result:
mark is activated, no change to shaping

Actual result:
The character at position 357 is no longer composed with the one at
position 358. (screenshot with-mark.png) whereas it was composed
before (screenshot without-mark.png)

The same thing works fine in a separate buffer in an RTL paragraph.