Bug (?) in compose

[Clock Source]

Good day!

Please get me a tips, where I must catch bug. Not all compose
combinations work. For example:

<Multi_key> <s> <s>                     : "ß"   ssharp # LATIN SMALL LETTER SHARP S
 - work, xev output:
KeyPress event, serial 39, synthetic NO, window 0x5000001,
    root 0x262, subw 0x0, time 2509703, (375,120), root:(385,201),
    state 0x0, keycode 0 (keysym 0xdf, ssharp), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 2 bytes: (c3 9f) "ß"
    XFilterEvent returns: False



<Multi_key> <f> <f>                     : "ff"   Ufb00 # LATIN SMALL LIGATURE FF
 - not work, xev output:
KeyPress event, serial 39, synthetic NO, window 0x5000001,
    root 0x262, subw 0x0, time 2593224, (321,71), root:(331,152),
    state 0x0, keycode 0 (keysym 0x100fb00, UFB00), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 3 bytes: (ef ac 80) "ff"
    XFilterEvent returns: False


and that work too:
<Multi_key> <percent> <o>               : "‰"   U2030 # PER MILLE SIGN

KeyPress event, serial 39, synthetic NO, window 0x5000001,
    root 0x262, subw 0x0, time 2799406, (319,50), root:(329,131),
    state 0x0, keycode 0 (keysym 0x1002030, U2030), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 3 bytes: (e2 80 b0) "‰"
    XFilterEvent returns: False


How I may solve this puzzle?

-- 
	Best regards, V.