The following compose sequences do not work in ibus:

    $ grep -i arabic.*ligature /usr/share/X11/locale/en_US.UTF-8/Compose
    # Arabic Lam-Alef ligatures
    <UFEFB>	:   "لا" # ARABIC LIGATURE LAM WITH ALEF
    <UFEF7>	:   "لأ" # ARABIC LIGATURE LAM WITH ALEF WITH HAMZA ABOVE
    <UFEF9>	:   "لإ" # ARABIC LIGATURE LAM WITH ALEF WITH HAMZA BELOW
    <UFEF5>	:   "لآ" # ARABIC LIGATURE LAM WITH ALEF WITH MADDA ABOVE

They are needed because the Arabic keyboard layout outputs UFEFB on some key:

    $ grep -i fefb /usr/share/X11/xkb/symbols/ara 
        key <AB05> {  [           UFEFB,                UFEF5,                  NoSymbol,            NoSymbol ]};  // ‎ﻻ‎ ‎ﻵ‎
        key <AB05> {  [           UFEFB,                UFEF5,                     U06AB,               U06AD ]};  // ‎ﻻ‎ ‎ﻵ‎     ‎ګ‎ ‎ڭ‎

but the UFEFB characters is not what is desired, what one really wants is U+0644 U+0627. But xkb keyboard layouts can only output one keysym when typing a key, not two. So compose was used as a hack to work around this limitation of xkb:

The keyboard produces UFEFB and then the compose support replaces  this with U+0644 U+0627.

This works when the compose support in Xorg is used but not when the compose support in ibus is used.

How to reproduce:

1) First show that it works when using the Xorg compose support:

Start xterm like this (to disable ibus and use the Xorg compose support):

    env XMODIFIERS=@im=none xterm &

Then in the xterm, type

    $ echo -n b | iconv -f utf8 -t utf16le | od -x
    0000000 0062
    0000002

and we see that the b produces U+0062, which is correct.

Switch to the Arabic keyboard

    setxkbmap  ara

type “arrow up” to get the echo -n b | iconv -f utf8 -t utf16le | od -x line back, go back to the b with “arrow left”, type b and now one gets:

    echo -n لا | iconv -f utf8 -t utf16le | od -x 
    0000000 0644 0627
    0000004

I.e. even though the keyboard surely outputs only U+FEFB, the Compose support of Xorg transforms this into U+0644 U+0627

2) Now repeat the same test using the compose support in ibus:

Start xterm like this to use the ibus compose support:

    env XMODIFIERS=@im=ibus xterm &

Then in the xterm, type

    $ echo -n b | iconv -f utf8 -t utf16le | od -x
    0000000 0062
    0000002

and we see that the b produces U+0062, which is correct.

Switch to the Arabic keyboard

    setxkbmap  ara

type “arrow up” to get the echo -n b | iconv -f utf8 -t utf16le | od -x line back, go back to the b with “arrow left”, type b and now one gets:

    echo -n ﻻ | iconv -f utf8 -t utf16le | od -x 
    0000000 fefb
    0000002