FVWM: left and right control keys behaving differently

I'm running fvwm 2.6.7-3 on Linux Mint 19.2, and have the 
following in my .fvwm2rc for scrolling with the keyboard:

Key Left	A	CM	Scroll -100 0
Key Right	A	CM	Scroll +100 +0
Key Down	A	CM	Scroll +0   +100
Key Up		A	CM	Scroll +0   -100
Key Left	A	CS	Scroll -100 0
Key Right	A	CS	Scroll +100 +0
Key Up		A	CS	Scroll +0   -100
Key Down	A	CS	Scroll +0   +100

Strangely, Left Control + either Left or Right Alt + either 
Left or Right Shift works as expected with the Left, Right, 
and Up arrow keys, but does not work with the Down key!

Right Control + Right Alt has the same problem (works with 
Left, Right, and Up, but not Down).

Right Control + Left Alt or Right Control + Shift (either)
also works as expected.

Can someone please suggest where I should start trying to
figure this out?

Thanks!
-mandar.

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Output of xev from 4 button presses given below:

KeyPress event, serial 36, synthetic NO, window 0x6600001,
    root 0x154, subw 0x6600002, time 278988216, (26,38), root:(114,182),
    state 0x0, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyPress event, serial 36, synthetic NO, window 0x6600001,
    root 0x154, subw 0x6600002, time 278988811, (26,38), root:(114,182),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyPress event, serial 36, synthetic NO, window 0x6600001,
    root 0x154, subw 0x6600002, time 278989234, (26,38), root:(114,182),
    state 0x0, keycode 108 (keysym 0xffea, Alt_R), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyPress event, serial 36, synthetic NO, window 0x6600001,
    root 0x154, subw 0x6600002, time 278989667, (26,38), root:(114,182),
    state 0x0, keycode 105 (keysym 0xffe4, Control_R), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False